I generally rate college groups and professional groups on the same scale (same with coed/one-gender/whatever groups) simply because I have heard some college groups that sound as good as pro groups.
For me a ten is the best a cappella I've ever heard - perfect in every way. A one is the worst a cappella I've ever heard. A five is an average track, pretty lukewarm. It doesn't bring me much pleasure, but it doesn't make my skin crawl, either.
Anything above a five I enjoy to various degrees...anything below, I would be likely to skip past the track. Generally, for me to pay cash money for a disc, it would have to be at least a seven or eight overall.
Personally, I don't think the numbers are a huge deal - when choosing a disc based on the reviews, I would pay more attention to the comments and try to get to know the reviewers' preferences a bit.