For:
Beautiful city. Very much so. Quite a few parks, lovely architecture.
easy to walk around (don't need car, in fact bus gates have been put into place to reduce the traffic in the city centre).
against:
expensive - you can live a little out of centre and find cheap accomodation, but the cheaper places can end up quite a distance from the centre as the city follows the valley to some extent.
pollution/congestion - in a valley, and for reasons that escape me the A46 bypass doesn't meet up with the A36, so all traffic from Frome etc goes across Cleveland Bridge just to get to the M4. Connect the two roads, it would be much better.
Jobs can difficult to find - depends upon profession, Bristol easy commute and I'm an an engineer so no real problems, but the Bath job market isn't the best.
Things others have said that I really don't see:
Nightlife OK to me, variety of pubs, some of which are full of students, some locals, some gastro, some spit-and-sawdust. Saying that, quite a few pubs getting poncified.
Rugby-boys - As Bath is expensive, a lot of people who live here tend to be reasonably well-off, which is only important if you think that higher income is a good reason to dislike somebody or not. I think students are more of an issue, and during term-time there are 10,000 or so in a city of circa 90,000, but depends on your opinion of students as to whether this is good or bad.