That's a mixed bag. If you know exactly what you're looking for, are buying a high-end bike, AND know how to do all the assembly and tuning work yourself, AND have all the tools to do it, you can get a great deal. However, when you factor in $40-$60 shipping, $40-$50 assembly/tune up, and not being able to ride the bike or get it fit to you, and you had better be getting a screaming deal to make it worth the hassle and wait time. Not to mention the fact that you won't have a warranty on a used bike like you will on a new bike. I've bought them and sold them both ways, and in this case a new bike from a shop is a much better way to go.
Eric