Via Dare, I found Rakesh Agrawal talking about GMail as a work tool. I mostly use GMail that way myself, so I had a look. One quibble on the downsides:

No offline access: I guess this isn't true because you can always use Gmail's POP support to download messages into an offline client, but then I'd lose that notion of having one single email box for everything. And I guess I also haven't felt the pain as much on this as of late because I haven't been traveling quite as much as I have in the past.

Well, not exactly. I use the POP interface so that I have normal client interaction, but I still have everything archived by Google. So I get the benefits of both - if I've aged off my mailboxes here on the client, I can still go to GMail and search.