I have been using it since 3.0 on my laptop (core2duo 1.66GHz, 2G RAM, with debian sid on top). It mostly runs WindowsXP guest with 512MB RAM, single processor (for application development) and I feel the performance is quite acceptable.
GUI can do most common tasks, and commandline tools can do much more. Technical user can probably figure out all common operations (by clicking around GUI, and run VBoxManage --help, that sort of thing) without looking at documentation . Also, I think the documentation is quite complete and user-friendly, comparing to other documentation for free software I have seen.