They make some good bikes. Look for ones with the 105 groupset or higher (Ultegra; Dura-Ace) and chances are, it'll be a fine bike.

A few tips for buying used:

Look up the bike in question on bikepedia.com and compare to what's being sold. A lot of sellers are inaccurate as to model year (whether out of ignorance or on-purpose)...you don't want to think you're getting a good deal on a '12, only to find out it's a 2009.

Ask for the bike's serial number, before you go to see it. If the seller hems and haws...it's likely a stolen bike. With both of my used bikes, purchased through Craigslist, the sellers emailed a pic of the serial numbers when I asked about them.

There are some good deals out there. I got my Venge [A $4300 bike when new, with $800 worth of upgrades] for $1800- a year old with 600 miles on it, and like new. It blows away anything new I could have gotten for $1800. Wait for something nice to come along...post questions here for opinions on value, etc. -but be ready to move quickly if you see something nice at a good price, as the good deals get snapped-up fast. (Unless you're in an area like mine...hehe...that Venge sat on CL for a few weeks, as I negotiated the price down, because few people around here ride; and it was right before Xmas)