I am using it, and the software is still pretty new. THis means that some stuff doesnt work as well as you would think, or that a plugin can jam up the whole system. I am expecting it to get better once Wordpress and Wordpress MU become one package. Other than that it is fine. It just takes some time to build community.
I have never used Buddy Press but have heard others talk about it and I don't get the impression its real good. I run a Phpfox community and its okay but none of them are perfect.
Some scripts you have to buy mods for or even a forum and others have terrible support, you get what you pay for in most. If you're really really serious I would avoid any free scripts. Just my opinion.