If I use IMAP to connect to Gmail from Mail.app, and have Mail setup to cache complete messages on my hard drive, then I get the best of both worlds; I can read mail offline, take advantage of Mail.app's for the most part superior user interface, and if I need to access my email away from my computer or want to use some feature of Gmail's web interface, my mail is still all on Gmail's server too.
This sums up my opinion perfectly, so I won't type it myself 🙂
A.