I played for a few hours last night and so far it's living up to my hopes. I only did a couple of missions, then spent most of my time exploring the world, and I can say that it feels much bigger while playing it than it did in the videos I watched over the weekend. My biggest fear was that they'd mess up the driving (I enjoyed GTA4's driving for the most part) but I think they nailed it. Driving is easier, but not arcadey, cars still have a good feeling of weight to them but have much better grip and tighter suspension. You can lose control and spin out, but not excessively so, as in IV.
Some advice: Don't try to climb Mt Chiliad in a pickup truck, use a dirt-bike. And be wary, those ninja cougars from RDR are all over the mountains.