Oh, how long have I waited for this! Almost three years ago, I blogged about the then newly-introduced POP access for GMail (German) and, to summarize it for you, my conclusion was that POP does not sufficiently support GMail's specialities, in particular the big storage capacity (downloading tons of mail is no fun) and tagging (which could easily be replicated in IMAP folders).
Now, three years later, GMail implements IMAP and with that, in my opinion, GMail has become a global player in email that now actually deserves the name.
IMAP features are mapped to GMail features reasonably, for example tags to folders as I suggested, or IMAP flags to GMail "stars".
To test, I connected to my account with Thunderbird, and as far as I can tell now, it works nice and quickly. Good job, Google!
According to Jean Pierre, IMAP is not enabled for all users yet, but I assume everybody will get the feature soon. Once you do, make sure to check it out!