I started using Gmail as my primary email application/service a couple of years ago and have gotten better and better about labeling and filing my email. The problem is that I still may archive something in a label/folder and then forget to follow-up so I am really not being very productive. I like that I can access my Gmail easily on all my mobile devices with the mobile client or even via the mobile web interface (like on the iPhone) with a 3rd option of using POP or IMAP. Joel at Geek.com put together a very helpful feature on Getting Things Done with Gmail that I plan to implement as we begin a fresh new year.
Joel laid out his techniques that he uses with Gmail to try to emulate some of the practices of David Allen's Getting Things Done philosophy. As you can see in the article, Joel uses four (or more) key Gmail labels and filters to manage his inbox. I'll have to ask him about his experiences applying these same technique on his mobile devices. Do you use a filtering/label management system with Gmail to be more productive as well?