GMail does a pretty good job of mapping tags to folders, but it ends up leaving edge cases that are hard to handle. Like how all emails will still be in the All Mail folder, even after you "move" them to another folder (tag).
I'm not sure there is any real solution, other than making mail via IMAP read-only.
My message counters are effectively random, my smart mailboxes no longer work, to the point where adding a new smart mailbox changes the behavior of preexisting smart mailboxes, it seems to have lost 1/200 of my existing messages, it can't move mail from folder to folder because of those dangling references, so I can't even break my inbox into smaller exportable folders, which would be handy because Mail 7 gets 2/3rds of the way through reindexing my inbox before crashing now. Junk mail randomly shows up in every folder.
It's like running pre-1.0 Ximian Evolution from back in 2000, all over again.
I haven't hit any major bugs, but my friend did. But they removed the "view different encoding" thing from the menu, or whatever it was called. Before you could switch how you view multi-format emails, and set plain text as default using a terminal command. I'm really sad to see that gone.
You can still compose and view plain text email - the only change is that it doesn't display the plain-text alternative when someone sent HTML. Given that they support both, this makes sense from a usability perspective and makes "removed support" read as hyperbole.
Today's new awesome Mail 7 behavior: reply-all to a mail with 10 recipients, get 10 new messages in my inbox all from me!
It was created by OS X Mail version 7 on Monday.
It's not out of the woods yet! This user now can't see any of their other folders when they try to set up an account with OS X Mail because it breaks on the INBOX.inbox.
I suspect again somebody was doing something clever with rules, and it's broken exactly BECAUSE OS X Mail has gotten better at server side namespace detection, and a folder name was saved as "inbox.foo" in the rule, but the INBOX prefix has now been implicitly added where it wasn't before.
I might be writing a third blog post in the near future I think!
Anecdotally, I use Apple's Mail.app for 5 different accounts containing over 50 GB of email without problems. Last problems I remember having were importing from Leopard to Snow Leopard – since then it's been smooth sailing. Admittedly, I use POP, not IMAP, so I'm not prone to the sort of bugs in the article.
I use/run cyrus-imapd for years and have never had an issue with mail.app. But it is pretty frequent that I hear people with other imap providers (especially gmail) that have issues with it.