I like this deal.

I know I'm one of the few, but I like what the Reds have done here. It's not easy to restructure a roster, but the Reds have needed to do just that for years. They began the process last year by trading away Adam Dunn and Ken Griffey Jr., but finally completed it by trading away Edwin Encarnacion. He was the last player who obviously needed to go.
For far too long the Reds were built like a beer league softball team. As the new defensive metrics continue to develop, they reveal more and more the true value of defense. Unfortunately, the Reds have been locked into a 25-man roster that largely prevented them from being a good defensive team. The presence of Adam Dunn, Ken Griffey Jr., and Edwin Encarnacion gave the Reds no roster flexibility and no chance at fielding even a competent defensive club. Finally, with this trade, the Reds no longer have any significant defensive liabilities.
Completely restructuring a 25-man roster is a significant challenge and, at times, it can be painful, but this was a necessary move for the Reds and one that makes them a better team in 2010.
The Players

The Reds traded Edwin Encarnacion, Zach Stewart, and Josh Roenicke to the Blue Jays for Scott Rolen. I still haven't seen the definitive word on whether money is changing hands in the deal, but if the Reds can get the Jays to pick up some of the contract then it looks more favorable for them.
Of course, the hue and cry in the Reds nation is that the Reds overpaid. Frankly, I just don't see it, as that train of thought is based on fans 1) overvaluing Edwin, 2) the prospects we gave up, or 3) both.
As it stands, Edwin simply has no trade value. In fact, in light of his escalating salary, he may have actually had negative value. When rumors of this trade were first raised in the press, it was clear that the Jays had no interest in Edwin. It's a reasonable inference that the real value in this deal for the Jays came from Stewart and Roenicke. Edwin went along for the ride to give the Reds some salary relief.
Offensive Impact

Unfortunately, Rolen isn't quite the offensive player he was once. A shoulder injury has robbed him of some of his power, but in 2009 he has made the adjustments necessary to be an effective offensive contributor. On the season, Rolen is hitting .316/.366/.471/.837 and while he has a .342 BABIP, his performance is not the result of luck, as he has a stellar linedrive rate of 24.9%. On his career, Rolen has a slash line of .284/.370/.500. And, while the big power numbers are unlikely to return, his overall production should be solid.
As for Edwin, his career slash line of .262/.344/.449/.793 has been characterized by massive slumps and never ending inconsistency. He has always appeared to have good upside, but is no closer to reaching it than when he first arrived.
If he can stay healthy, Rolen should be an upgrade on offense.
Defensive Impact

In 2009, Edwin has sunk to a whole new level on defense. I didn't think it was possible to get worse at the hot corner, but he has managed to do so. I really don't think most people appreciate just how detrimental his defense is to the performance of the team.
In 2009, Edwin has a UZR (Ultimate Zone Rating in Runs Above Average) of -6.0 and a UZR/150 (UZR Runs Above Average per 150 Defensive Games) of -23.1. His career UZR/150 is -12.2.
He doesn't fare any better under John Dewan's +/- system. In 2009, Edwin has a -10 rating, which translates into -8 Runs Saved. In a mere 362.1 innings, Edwin has cost the Reds 8 runs more than the average third baseman. That ranks him 30th among third basemen. Basically, he's dead last among 3b, despite having missed a significant amount of time with injury.
On the other hand, Rolen is still stellar with the leather. He's not quite what he used to be, but he remains a strong defensive third baseman.
In 2009, Rolen has a UZR of 4.8 and a UZR/150 of 7.9.
Under the +/- system, Rolen is a +18, which translates into 14 runs saved. So, he's been 14 runs better than the league average 3b.
Defensive metrics still aren't without flaw, but when the findings of one are supported by another, then you can feel more confident in what they reveal. If you look at these measures, then it seems fairly safe to say that the Reds are going to save roughly 28-30 runs on defense by going from Edwin to Rolen. Give or take, Edwin is ~15 below average, while Rolen is ~10 runs above average.
As a general rule of thumb, a win is considered to be worth ~10 runs. That number is arrived by looking at all the teams throughout baseball history who have scored one more run than they have allowed per game, and finding that they have a .600 win percentage. In addition, the teams that have allowed one more run than they score have a .400 win percentage, which means that each additional run leads to .100 additional wins.
If Scott Rolen can stay healthy and play something resembling a full season, then speaking strictly from a defensive point of view the Reds are improving in the Win column by ~2.5 wins.
Departing Prospects
Most of the complaints about the deal focus on the loss of Zach Stewart, and, to a lesser extent, Josh Roenicke.

Stewart really opened some eyes this year, as he proved to be much more than a raw, live-armed college closer. He made 14 starts between high-A and double-A ball before being promoted to triple-A, where he was shifted back to the bullpen to limit his innings pitched.
For high-A Sarasota, he posted a 2.13 ERA, 1.30 WHIP, and a 32/8 K/BB in 42.1 innings pitched. In addition, Stewart had a stellar 2.42 GB/FB ratio. He was quickly promoted to double-A, where he kept on rolling. He posted a 1.46 ERA, 1.05 WHIP, and a 31/10 K/BB ratio in 37.0 innings. He also continued to pile up the groundballs, posting a 1.96 GB/FB ratio.
As for Roenicke, I like him more than most. He's got a very live arm, keeps the ball down, and gives up few walks. He's an intriguing power arm with the potential to work high leverage innings. In the minors, he had a 2.57 ERA and 32/6 K/BB ratio in only 28.0 innings. Roenicke also induced his fair share of groundballs, as evidenced by his 1.60 GB/FB ratio.
Not only did both prospects have good numbers, but they also have plus stuff. Neither relied on smoke and mirrors to post their production, so these certainly were legitimate prospects with value.
Final Thoughts
Ultimately, I think Rolen will have a very positive impact on the Reds in 2010. His defense alone will be a significant improvement and he should also provide the type of quality righthanded bat that we have lacked in 2009. When paired with Brandon Phillips, a healthy Scott Rolen should work to counterbalance the lefthanded bats of Joey Votto and Jay Bruce.
Unfortunately, the Reds had to give up Roenicke and Stewart to acquire Rolen. I suspect that Roenicke had to be included to get the Jays to take Encarnacion. By including Edwin in the deal, the Reds save $4.75M in 2010, which effectively reduces Rolen's cost next year to $6.25M. Early reports are that the Jays are picking up the rest of Rolen's salary in 2009, so the monetary implications of the deal are fairly good for the Reds.
Of course, it would have been nice if the Reds could have acquired Rolen without giving up Roenicke or Stewart. However, given that the Reds already have a lot of good young talent on the team and in the rotation, it hardly seems unwise to deal away prospects for established veterans who can help at the MLB level. The purpose of the farm system is to support the MLB team, which can be done both by promoting prospects or by trading them away.
Stewart certainly exploded onto the scene in 2009, but to me his ceiling seems to be that of a #2 starter and he's no sure bet to reach that ceiling. In fact, I wouldn't be at all surprised to see him top out as a #3 starter or even a closer. The idea that he's a future #1 starter or Cy Young winner based on a mere 14 start sample size seems more than a little overly optimistic. Personally, I wish we could have kept Josh Roenicke, as he is the more likely of the two to contribute at the big league level in 2010. Still, given the volatility of relievers, they are usually fairly easy to replace. However, I fear that may not quite be the case with Roenicke.
Ultimately, there is a great deal of inherent risk in pitching prospects and Stewart is more likely to end up a #3 starter or worse than he is a #1 starter. In addition, I still have a question or two about his mechanics, which may reduce his ability to handle a 200+ inning workload on an annual basis. To me, Stewart would have to develop into a legitimate #1/2 starter for me to lament his departure and I question his ability to do so.
Despite their recent struggles, the Reds seem rather close to being competitive. By adding a veteran righthanded bat and improving the defense at the hot corner, the Reds should be much improved in 2010. After the Rolen trade, the Reds seem to be an impact bat and a healthy pitching staff away from being a tough team to handle next season.
I'd be surprised if the Reds didn't extend Rolen's contract to keep him in Cincinnati beyond 2010, so if he can stay reasonably healthy over the next few years, then the Reds should be a much improved team. The Reds certainly gave up good talent in Roenicke and Stewart to do it, but a healthy Scott Rolen puts the Reds one step closer to a postseason birth.