While K:BB is a good measurement, you have to combine it with WHIP. There are those pitchers who will strike out 2 for every walk, but still give up 2 hits per inning.
This is how I do it:
1) Look for pitchers with a low WHIP. If it's a starter and they have a low WHIP and a high ERA, don't worry about it.. they're the victim of bad luck (unless they have a very high HR : IP ratio).
2) Of those with a good WHIP (Starters <1.20 and Relievers <1.10), check out their K:BB to figure out which ones are the better ones.
3) After that, check out their K/9 and Innings per Start. Make sure they're not throwing too many innings (or pitches) in concecutive starts (like Matt Morris last year). For relievers, check out their appearences/ week... look for guys that see the field a lot.
Basically it's WHIP, then K:BB then K:9, and I look at ERA last. If you see a guy (e.g. a starter) with a very high ERA after 5 starts (say 7.00+) but a WHIP in the 1.05 range and a K:BB of 3:1, he'd be a great guy to target for a trade, since this is a guy who will probably get better.
Just my oppinion, and remember I'm the guy who drafted Freddy Garcia...