Yes, 3.0 is playable. In fact, I refuse to convert my old 3.0 game to 3.5. I also still love a couple of the 3.0 spin-offs: Omega World d20 (brilliant!) and Judge Dredd d20 (honourable mention). I would be more likely to keep 3.0 than 3.5. But, I would run a 3.5 game, too. Core books only...