If you can snag a good used bike that's been well maintained, go for it. Make sure you give the frame itself, especially if it's carbon, a very good once over. I think the biggest downside is that you don't have the strength of a warranty. Plus, there's always the chance it was in a crash or compromised in some other way.

As for steel, they're hard to come by anymore. Again, used is an option. You might take a look at Jamis and Bianchi for new. What size do you ride?