I think that this movie will be fine. Since Guillermo Del Toro's directing, it's going to have Ron Perlman(good). Also, Tom Cruise may be a Scientologist douchebag but (like Ross said) he can act when he wants to. Collateral, Last Samurai, etc...
I think that Del Toro's enough of a fan of Lovecraft to do this story right and give the Old Ones the justice they deserve. I don't see any way that this movie could honestly be -that- bad.
Everything Del Toro's touched I've liked.