About a month ago I swapped over to using GMail as my primary email account. I've been very impressed with it, as I've ad bad experiences with web-based email before. I've always had my email infrastructure set so that swapping should not be the hardest mission in the world...I have a number of "public" email addresses that are typically forwarders to a single inbox, so changing over was mainly a technical matter of repointing the forwarders and letting a couple of people know. I'd had the GMail account set up into Thunderbird et al for a couple of years previously, so that was all ready to go.
One additional thing I did was set GMail up to also pick up the old account. That way I could remove it from my updates, and ensure that any replies I did to those emails came from the new main account. All the PC's I use also now have the Notifier installed, to let me know when messages are received (funny when 2 are sitting next to each other, and both bleep at the same time...)
One issue I've always had previously is spam. I get lots of the stuff, mainly due to the fact that I have catch-all's on my domains. Thunderbirds spam filter really struggled to ID and clean it all up, and I often had to can through and flag the various offers of crap I got. GMail has managed to pick up about 95% of everything coming in, and cheerfully assign it to the bin. Its also let me see for the first time exactly how much spam I get..in the last calendar month I've currently had over 11,000 messages flagged as spam! That's getting close to 400 a day. In comparison I've had about 300 valid messages...
Once I got used to the "conversation" display of emails its very intuitive, and save a lot of searching through inboxes. I've also set up a number of rules to flag different types of emails under different categories (based on certain keywords or senders), and this lets me just change the view to emails regarding a certain subject..again very handy!