I know the catch (Score 1)

The catch is that if you use POP3 (or IMAP for that matter), you lose all the nifty features of their web interface. Namely:
- Ultra-fast search.
- Labels.
- Conversations.

I use those features extensively, so even with POP3 or IMAP available I would still use the web interface, which is the best one I've seen so far.

As an added bonus, I can access all of my stored email from anywhere. The ads are non-intrusive enough so that I never look at them anyway.