you'll wish John Connor went back in time to stop this game from being made
I'll go into more detail about each aspect, but the overall needs to be conveyed first to fully understand how bad this game is; its not finished.
Honestly, there are just too many glitches, flaws, problems and oddities that the only explanation I can suggest is that the devs didn't have time to finish the game, and they threw out an unfinished product.
If you're still interested after reading that, here goes......
Gameplay wise, this game is simply horrid. It tries hard to copy the duck and cover third person shooter stylings of such games as Gears of War, Dark Sector and Uncharted. The difference is, those games were fun to play. Terminator Salvation isn't. The game is as cookie-cutter and generic as they get. You move from area to area, enter a semi-open space, kill a few enemies, watch a horribly dull or completely pointless cutscene, and repeat ad nausium. Thats the entire game. The gameplay isn't even fluid. You'll have to adjust the sensitivity up to 80% or higher just to get the gameplay at a functional level, and after that you'll ask yourself why you bothered. The cover mechanic barely even works; it doesn't allow you to pick up ammo while crouched most times, and there is zero realism to it; just the same few cars and crates that always seem to be conveniently placed. This won't bother you much however, because the AI is so atrociously poor, you often won't even need to use cover, you can just stand behind a pole or car without worrying about the duck manuevre at all. Naturally, they throw in a few standard sections of the gun-turret or vehicle-mount variety, but nothing you haven't seen in 100 or so superior games.
Design-wise, the game is a total letdown. Level design is piss-poor, linear, and utterly predictable, and the environments are atrociously bad. Its like each little section was set up perfectly against the enemies by containing perfectly spaced out barrels, crates and cars to hide behind. The AI is horrid, as the enemies consistantly walk back and forth along instantly recognizable and predictable patterns. There are even glitches and oddities, such as enemies getting hung up or stuck on/in walls, and sometimes you just randomly find sweet spots where you can just stand and shoot at an enemy without it reacting until it dies.
Graphically, the game ranges from somewhat adequate, to terrible. Some of the weapon effects and cutscenes look decent, but there are sections where the graphics simply glitch, and even some areas where entire random sections of floor are just black, as if the game is struggling to fill in its textures properly. Honestly, this game looks and plays like an early PS2 title, and its not even as good as most of those were. Character models move very stiff and unrealistically, to the point where they look more robotic than the terminators. Sound effects are done well to be honest, with the instantly recognizable terminator music and sound effects serving as the only true and believable tie to the series. Unfortunately, this is ruined by the dialogue and voicework, which can only be described as sub-amature.
Unfortunately, the story does little to aid the sub-par gameplay. Its not that the story doesn't have its merrits, in fact its quite terminator-esque, its just that the execution just isn't there. The bad dialogue, ugly graphics, linear design, and unfinished feel of the game destroy any positives that might have been otherwise found in the game's general story.
The good news is, you won't have to deal with the ugly graphics, horrid gameplay and uninspired design very long; the game maxes out around 4 hours tops, maybe 5 if you've never played a 3rd person shooter before. Anyone who's played a shooter before and doesn't select the hardest difficulty will walk through this game in a single sitting with little to no effort needed.
This actually leads to the one and only positive I can offer; you trophy collectors will pick up 11 gold trophies and the platinum on one single 4-5 hour run, so if you're looking to pad the trophy stats, this is worth renting, assuming a platinum is worth the $6 to you.
Needless to say, there is absolutely no replay value to be had whatsoever.
Theres nothing else to say folks, this game is an epic fail.