I just switched off GMail's priority inbox after months and months of forgetting about previously read important emails.
Big problem: When you ALSO view your mail on you iPhone, it syncs read messages back to my Priority Inbox organized GMail, and since it's not always convenient to respond from your iPhone, half of your mail will get lost. Unless you remember to mark messages unread.
Anyways, priority inbox seems like a good UI idea. Why not get to the important stuff first? But if you aren't meticulous in your organization it will help you on your way to speedy disorganization.