It's obviously not up to the standards of GTA since you're always on foot (or in the air), but it's definitely a huge map for those purposes. And Mark Hamill voicing the Joker is always worth the price of admission all by itself.
There was always something hilarious about riding your horse through the woods in RDR and then suddenly being pulled off of it by a cougar or bear. I enjoyed RDR quite a bit, but at the same time, I haven't yet found myself going back to play it a second time.