Well, I bit the bullet and bought the game today. Tooled around a bit trying to get used to the driving mechanics, the aiming/shooting mechanic still has me a bit thrown for a loop. Haven't gotten much into the story yet just been trying to have fun once I could. Good looking game so far.
While the map is open from the get go I've found it pretty satisfying to stick to the game's mission structure for the most part and it gradually opens up new areas to you so there's always something novel around the corner. The game says I'm like 30-something percent complete but I still have a lot of unexplored spots in my map which I kind of like.
It also gives the characters a sense of space which I think adds something when they cross over into the other's world. I actually felt kind of excited when Trevor left the boundaries of his area, you really could feel things were changing.