Not as bad has been made out
I'm glad I rented it, because I sat down at noon and it's half 4 now, game finished. But if you do rent it then it's a great rent. You don't have to worry about having enough time to play it, you will get it done.
I didn't find the action as repetitive as some people said, mainly because, with the diverse environments, the machines don't make it easy for you to flank them. I think the machines are pitched just right (the "wasps" are annoying but during bigger fights you can ignore them, where as the T-600's are very tough, but can be taken down with the right weaponry and maneouvres) and I actually think the storyline is better than that of the film.
I also didn't feel the title was lacking drama and felt compelled to crack on with the game, and enjoyed the on rails sections. I completely wasn't bored. I am a massive Terminator fan, admittedly, but in all honesty without this game being an essential purchase a la Assassins Creed 2 then it was only going to draw in Terminator fans anyway. Don't buy it, cos 4 hours later you will be sat there wondering where you £40 has gone (or even you £10 if you go pre-owned), but if you are a Terminator fan and have a lovefilm account or even still have a blockbuster video nearby, give it a bash and enjoy it.