Rodriguez's effort is far more entertaining and I think also the better film. It's funny and insanely violent. Tarantino's begins really well but just becomes annoying in the 2nd half. There's a very long car chase which admittedly isn't really my thing, and the characters in the 2nd half are probably the most irritating of any of his films(which is saying something). But the first part of his film is pretty much at the same cinematic level as RR's one.
Some of the fake trailers are wonderful - particularly Edgar Wright's and Eli Roth's. They really capture the aesthetic that the project demanded. One question that I haven't seen addressed is why Tarantino's film starts with that idiosyncratic Grindhouse look(damaged film, sound outages etc) but then transforms into looking like a normal 2007 Hollywood film after the halfway point. Maybe it's a fault with this disc, as I can't find a single review even mentioning it. Would love to know.
There's loads of extras which is obviously a good thing but some of them get a bit repetitive, especially those focussing on the actors. Actors are always annoying. The best ones are the various commentaries (including those for the fake trailers) and an hour-long interview with RR and QT on stage with the New York Times. Warning: There's a horrible bit in the 'making of' documentary for Edgar Wright's fake trailer where he talks unironically about 'retards' and 'mongoloids'. Have to say I've lost a lot of respect for him because of that. Very surprising.