Personally, I keep stumbling across 'great deals' on used engines, but I can't bring myself around to buying one. Not only is there the issue of the condition of the engine, there's also the lack of warranty/support. There are some decent .60 size engines out there for under $100, and I think I'd be inclined to go with one of those.

These are my own personal thoughts, and I'm sure many people have had good luck with used engines. Headaches and dead sticks are just some things that I like to avoid at all costs.