St Catherine's is the largest undergraduate college, so if you want to avoid studying at a tiny college, it's a good choice. It's also probably the most progressive, so you don't have to deal with the stereotypical Oxford elitism. You're guaranteed accommodation for all three years, and the facilities are pretty good. The JCR is one of the best, in my opinion, and it feels much more like a bar than a dusty basement (which is pretty common for other colleges).
As for the cons, most people think it's really ugly, and it's a lot less central than other colleges. Oxford is still very compact, but it is a bit out of the way. We do have two college dogs though, and they're really cute, so I think it balances out.