Here are this year's percentile breakdowns:
text
The percentiles are a bit higher than the last results posted on Wiki from a few years back. i.e. I think 700V was in the mid 80s there and 800Q was in the high 80s.
So either people got dumber or the test got harder... but it worked out great for me
Edit: and a "good" score depends on the school you're applying to & your field of study. Schools should report average scores, and so long as you're around there, that's OK. GREs don't get you into grad school, but performing poorly will keep you out.
Edit2: My undergrad advisor says he rarely votes to allow anyone below a 750Q, arguing that then you're going beyond careless mistakes into the land of ineptitude. This is aerospace engineering.