It depends on what you're doing. But there's a whole world out there of people being very productive on 4GB MacBook Airs, and Ubuntu uses far less memory than OS X.
Yeah, it's enough for me. It's my primary development machine by choice, and it's enough to do anything I need at a terminal (running emacs, developing and testing rails servers, docker builds, etc.) while keeping Chrome open with a few dozen apps including a few gmail and chat windows open.