Works as advertised. A little slower than my local ISP IMAP/SMTP servers, but not too bad, and because Thunderbird will happily wait for a server while you do other stuff, it's not painful at all.
Gmail's tags don't quite translate into IMAP folders, but it's probably close enough.
Conversations display as separate email messages in Thunderbird. You can use View / Threads to make it similar. Filing stuff in Thunderbird only does one message though, whereas in Gmail that'll do the whole conversation.
Deleting from Thunderbird moves it to a folder (eg gives it a Label) called [Imap]/Trash — which is how it appears over in GMail. Ideally it would move it to [Gmail]/Trash which seems to match the “real” Gmail Trash, but TB doesn't have that option.
Sent messages by default go into the TB folder Sent, but this can be changed to match Gmail's [Gmail]/Sent Mail in TB: Gmail account properties; Copies & Folders; When sending messages; Place a copy in: Other / Sent Mail on Gmail.
Moving messages to [Gmail]/All Mail appears to be the equivalent of pressing the Archive button in Gmail.
All in all, works well.
(Reminder: Gmail IMAP is rolling out this week. If your Gmail preferences say “Forwarding and POP/IMAP” then you've got it. At present you'll need to switch to US English for it to be given to you.)