I'm probably only thinking about John Smoltz because his ups and downs have been a part of my fantasy team, but I love the pickup by the Cardinals. The Red Sox were in a tight race and couldn't afford to let Smoltz cost them even one more game. The Cards, on the other hand, are a virtual lock to win the NL Central, so Smoltz is a gamble worth taking. 9 strikeouts in 5 innings is impressive, even if it's against the Padres.
Will Carroll (subscr. required) wrote that the biggest issue in regards to Smoltz is his recovery time between starts - he may not be able to get up to full velocity on only 4-5 days rest. If that's the case, it wouldn't be that hard for the Cards to get creative with the expanded September roster and just have Smoltz pitch once a week the rest of the way. And if that turns him back into a solid starter, he could be an effective weapon as a sort of well-rested 4th/spot starter in the playoffs, where there are lots of off-days.
The Cards have 3 very good starters for the playoffs - Carpenter (2.16 ERA, 4.8 K/BB ratio), Wainwright (2.61, 2.8, and an excellent track record in the 9th inning of deciding playoff games), and surprising Joel Pineiro (3.15, 4.4).
No improvement needed there, but replacing Kyle Lohse and his 4.67 ERA with a well-rested Smoltz would give the Cards four strong pitchers and make them tough to beat in October.