The game has two gameplay modes: single player and co-op. I've only tried single-player, in which you play John Connor making his way around Los Angeles shooting at machines, together with several NPCs. The game is played from a third-person perspective, which I'm not too fond of. The game has three levels of difficulty; I played at medium.
Lets start with the positive aspects first. The graphics is what you might expect from a game with hardware requirements such as these. The cutscenes are nicely shot with cool camera-focus effects, and the overall environment is impressive. (My HD4870 had no problem running it at 1600x1200 at maximum detail.)
The designers have tried to avoid the regular arcade shooting and introduce some tactics by letting the player take cover and move from cover to cover. Some enemies can only take damage from behind, so you will need to move carefully around these enemies. The endo-skeletons carry miniguns which inflict serious damage, so using covers is necessary in order to stay alive. Also, if any endo-skeleton gets to close, it will grab you by the neck and throw you head first into the nearest wall. You can only carry two weapons at a time, so you need to think a little before switching weapons. Weapons and ammunition are rather plenty, even if I ran out of ammo at some places.
There are several places where you get to play gunner in a vehicle, but boringly enough you don't get to drive anything (at least in single-player mode). Unfortunately, none of the vehicle parts presents much of a challenge.
Which brings me to what probably bothers me the most with the game. There is really no challenge. I played the game from beginning to end in 5.5 hours (according to Steam), and long sections just felt like transport; another street with endo-skeletons and other enemies which are taken care of by suitable weapons. The "crawl around to shoot the spiders from behind"-routine quickly becomes exactly that, routine. There are also remarkably few enemy types: Aerostats (small flying things which shoot at you), Spiders (walks around and shoots at you, needs to be taken down from behind), Endo-skeletons (walks around and shoots at you, needs a grenade or rocket launcher to take down), well, that's it. Oh, yes, and some form of driver-less motorbike in some of the vehicle chase scenes.
If I pay €40 for a game, I really like it if it lasts more than 5.5 hours. Or at least presents me with a good challenge, or has a intriguing atmosphere or gameplay. Even Half Life 2: Episode One and Two gave more gameplay for the money, despite that they were pretty short.
Steam's metascore gives 56/100 to Terminator Salvation, which I think is a little on the low side. I give it a 65 for good presentation and some fun arcade shooting.