I've experimented with GMail's Priority Inbox since its release. I can say this much - I hate it. It seems to consistently tell me about mail I don't care about, and leave out mail I do care about. I tried training it to no avail. Oh well. Just my opinion and YMMV.
What I do like is the Multiple Inboxes Lab feature. That allows a quick display of your inbox, plus starred items and drafts to the right.
I like even more the Undo Send feature, which went from the Lab to an integral function. It now allows up to 30 second undo. This is useful for people like me who hit 'Send' before reading the email ;p. For the majority of emails I 'undo' the send to correct grammar or other things (yes, I'm that picky). It used to be limited to 5 seconds, but now we have a full 30 seconds.