I haven't had a lot of time to play (PC with an Xbox360 controller) and I have focused on single player, but here are my reflections on the game.

Vehicles - The cars feels a bit loose to me as the slightest movement of the stick makes the cars turn and you have to correct which causes the cars to snake across the road. The helicopter I managed to fly from A to B, but it took a while to get there as it felt much slower than driving from A to B even though you were going in a straight line.
The airplanes I had a really hard controlling at all, most attempts ended up with a crash, yet still in some strange way we all survived all of the time.
The dirt bikes, WOW I really like the controll I had when going at full speed. No snaking across the road and they turned nicely, BUT how come you can drive at full speed straight into a concrete block without the slightest damage to the motorcycle or yourself? The cars take damage from running into stuff, but not the bikes? The only time I crashed was going cross country driving off of a cliff landing on the front wheel and was thrown over the handle bars.

A.I. teammates or as I prefer to call them A.D. (Artificial Dumb*sses) teammates. When getting on a boat one guy will run off of the jetty into the water, swim around to the other side of the boat and then climb back in. And when exiting the boat atleast one guy and sometimes even the main character will jump into the water and swim to the jetty instead of going on to the jetty.
I would also have liked to have the ability to give individual commands (as in GRAW), choose individual loadouts and do custom modifications for each teammate.
Also, if I choose to get on a bike the A.D. teammates will just stay where they are and then magically appear when I get off of the bike. Why not have them get into a car and follow me?

A.I. opponents also seem to have somewhat limited intelligence. I was doing the mission where you have to kill the three instuctors and decided to try to snipe one guy from about 550 meters, which took me about 28 tries before I succeeded, and even though the enemy was alerted 27 times by my misses they never found me as they would always send the helicopters and foot patrols to the same place.

Leaving the game: If I choose to Exit to desktop the game would become unresponsive when I clicked Accept and I would have to force quit the game from the task manager. If on the other hand I first choose Exit to main menu and then Exit to desktop the game exits properly.

Over all I had a good time playing the open beta, in spite of the slightly annoying microstutters that occur every now and then.