The Cubs made out well, giving up a good reliever who is a year from free agency and getting back a big league starter and two mid-level prospects who will have major league value. The Reds, meanwhile, continue what I can only assume is a rapid emptying of their farm system to try to win again in the two years before Joey Votto reaches free agency and/or Dusty Baker shreds every last arm on their pitching staff.
Marshall has found his niche in the bullpen over the past three years, posting a 2.61 ERA with 203 strikeouts and 60 walks over 189 innings and allowing just seven home runs. (He allowed seven homers as a starter in 2009 across 43 innings.) He hits 88-91 mph with his fastball that has a little glove-side run but works primarily off his two breaking balls, a huge upper-70s curveball that he throws more than any other pitch and a mid-80s slider/cutter that's effective because it's so close in velocity to the fastball.
Pitching in relief allows him to limit his use of the fastball, but as a starter, he'd have to change his approach. The fastball would likely be 86-89 mph without enough life to get away with it. He's the rare reliever who's worth more than two wins above replacement and, barring injury, should provide that kind of value to Cincinnati. However, given how much he's pitched (158 appearances in two years) and the general attrition rates of relievers, there's risk for the Reds even with just one year of control.
(Funny side story: I remember our Toronto draft-room debate over Marshall in 2003. I hadn't seen him, since I was handling statistical analysis only at the time, but he rated very highly in my system after a strong junior year at VCU. The scouts argued that his thin frame wouldn't hold up as a starter in pro ball and that he'd likely lose velocity pitching every fifth day. To some extent, this was true -- Marshall isn't a starter. What we never discussed, as far as I can remember, was what he might look like in relief other than assuming he could be a lefty specialist because he had a good curveball.)
Wood leads the trio coming back to the Cubs as a solid back-end starter with a chance to be more if he alters his pitch selection. Wood has four pitches, but his worst offering is his four-seamer, which he uses more than all other pitches combined. His 86-88 mph cutter is going to be much more effective because it moves in on right-handed batters, while the four-seamer is flat and too easy to hit in the air. His 74-78 mph curveball is sharp, a two-plane breaker with pretty good depth, and his upper-70s changeup is solid-average due more to action than arm speed.
Wood was overworked by Dusty Baker -- I know, I'm as shocked as you are -- in 2010, throwing more than 200 innings across all levels despite a small frame and no history of pitching past Labor Day, and that also likely contributed to his rough 2011. Managed properly, with more emphasis on the off-speed pitches, he could be a league-average starter, and he has five years of control left, two of them at the minimum salary.
Sappelt looks like a very good extra outfielder. He has a simple swing, short to the ball with good use of his lower half, but he can't handle center field except on an emergency basis, and his size and swing aren't going to produce the power to profile every day in left. He makes a lot of contact, however, and I wouldn't be surprised to see him hit .300 -- but without the OBP or power to make him a regular.
Torreyes is a tiny second baseman -- Baseball-Reference has him at 5-foot-9, 140 pounds, and I would bet he's shorter than that -- but he has two above-average tools: hitting and running, with good bat speed and a simple swing for high contact rates. He could end up an average regular at second, but he is probably three full years away from the majors. Wood alone justifies this deal for the Cubs, but the chance that Torreyes becomes an everyday guy turns it into a potential big win.