Some actors shouldn't even try accents. Take Kevin Costner, for example. Watch his feeble attempt at a British accent in Robin Hood: Prince of Thieves (he actually gave up on the accent less than a third of the way through the movie.) He did slightly better with a Boston accent in Thirteen Days, but even that was annoying at times. His best work usually comes when he speaks naturally.
So if you come see me in my next Shakespeare production this May and you happen to notice my Bronx accent, don't be too harsh. It could always be worse. :)