In many ways Mamet has a similar problem of making a lot of characters sound like himself, but he's certainly had his moments of brilliance, and Redbelt is certainly a step up for me. 

Tarantino has had moments of speaking for his characters in the past, including Reservoir Dogs but in previous films he's made up for it with a much more solid plot. Death Proof was neither clever or particularly compelling to make up for it. 

That said I still enjoyed the film and liked it on its own merits, but Tarantino has built one of the most impressive filmographies of his generation and anything less than Kill Bill/Pulp Fiction will seem like a step down and a bit disappointing.