As far as modelers go, it's the proverbial REAL DEAL - I know a guitar player who literally replaced a "fridge" full of high end rack gear with one Axe-FX.
That said, as awesome as it is, it might be overkill in 80% of WORKING bass player situations. It's also very deep to program, with a big learning curve (well, at least to get the most out of its capabilities), and it's very, very expensive. I'm sure it'll sound great on bass, but honestly, I'd probably use only two or three sounds. Then again, YMMV.
New York Bassists club member #9