So I've been using GMail for a little under a month now and I am still very impressed. I am still unsure how it would fit into a larger organisation (with 10+ seats) there are 2 things I dislike about it at the moment.
Firstly there's the username issue, I finally managed to get my username straightened out and in fairness I may have been partly to blame. I had assigned my required forename.surname as an alias to my current account, so I was unable to rename until I removed this alias - so after waiting the required 5 (or was it 7 days? I actually waited 14) I finally started to rage at Google, hammering my keyboard with search terms until I finally gave up and decided to mess around with my account settings the error message in a round-about way lead me to believe removing the alias would work, and it did!
Secondly I don't like how the username you sign into email with follows you across all of Googles sites. Now for most people this isn't a problem and its actually benefits them but for me it doesn't. For my personal business I have my own domain and therefore use that for all my email, I have a separate signon for Blogger and for YouTube (although I think I somehow ended up managing to link the Blogger and YouTube accounts)
It's not really a problem, but it is an annoyance.