I've been a big GMail user ever since I got my first account and I've gotten very used to it. About a year ago I switched the email for my website and our corporate email over to Google Apps and eventually was able to drop Outlook completely .
I've been extremely satisfied with this setup and don't regret it at all. However, as much as I like GMail and all, it's sometimes a drag when actually editing new emails or responses to existing ones. It's not really GMail's fault; it's just that a TEXTAREA isn't really a good editor for email (I try to stick to plain text email only as much as possible).
Fortunately, I discovered a while ago the It's All Text! extension for Firefox. This little extension gives you a small button next to textareas; when you click it, it will open your favorite text editor to edit the text, and then replace it with whatever you saved before closing it.
Now, I can just use this to edit complex/longer emails in gVim, and get all the text-editing goodness I'm used to!
Firefox did have one nice feature I missed at first: Spell checking. This wasn't always useful, though, as it only really supports a single language at a time, and I regularly use both Spanish and English, but it was better than nothing.
Vim, however, also supports spell checking! I just had never bothered setting it up correctly. I've now configured my .vimrc file so that I can enable/disable spell checking for English or Spanish with a few key-presses. Here's how I configured mine:
map <Leader>se :setlocal spell spelllang=en_us<CR>
map <Leader>ss :setlocal spell spelllang=es_es<CR>
map <Leader>sn :setlocal nospell<CR>
Now I can just press \se in normal mode to start the spell checker in English or \ss for Spanish, and then \sn to disable it later on, if I want to. Otherwise I just keep spell checking disabled so that it doesn't get in the way of my coding sessions.
The combination of It's all Text!, Vim and its spell checker makes for a much nicer GMail experience for me.
 I still need to migrate some old personal email over to my GMail account, which I haven't done because I'm lazy, but I should definitely get around to that some day.