Singleplayer is absolutely astonishing. Great graphics, perfect atmosphere, solid gameplay, best random events in an open world, etc.. It's a Western fan's dream game, and a must have for anyone that enjoys open world games.
The multiplayer Free Roam on the other hand has some MAJOR problems that need to be patched. The invisibility glitch is absolutely horrid. Your weapons, your mount, your character, and sometimes all 3 at the same time randomly disappearing is not cool. It affects every player randomly as well. Some people see you, others don't, some see your horse, other don't, etc. This is a huge annoyance. It's not just in game either, everything in the outfitter is ALWAYS invisible when I try to access it from a public match.
On top of that are the network errors that seem to stick with you once they happen until you restart the console (360 specifically). This is just as annoying, but I'll chalk it up to launch day overload. The invisible glitches HAVE to be patched though. The Free Roam rooms just can't stay full because people are constantly leaving to reset their character, or getting kicked.
Competitive multiplayer on the other hand is a LOT more solid. Movement fluidity and controls still aren't at the level to compete with the likes of Uncharted 2 multiplayer, but it's still fun. None of the invisible glitches seem to happen here, and network errors seem much less prevalent. I was playing with the same group of people for almost 2 hours with no problems.
One plus that is good all around is the quick loading, especially when compared to GTAIV. Menus are snappy, and loading the singleplayer after a failed multiplayer sessions happens super quick. Saving, and changing attires is also instant. Good job there. If they can fix the Free Roam glitches, this will definitely be a winner from all angles.