I finished it on the weekend. It's short and underwhelming, but not completely unplayable. The first issue with it is the story - as a prequel to the actual film, it's not particularly interesting on its own (you go a rescue some people trapped in Skynet territory and that's it.) Next issue is its difficulty - it can be quite cheap at times. Best example of that is when you're hiding behind some cover, a Terminator comes and punches you for a one-hit kill - even though its not physically close enough to do so. I liked the turret sections though - but I'm a sucker for those.
Ultimately, I did enjoy it. It reminded me of older 8-bit/16-bit game design where every enemy had a certain strategy to deal with them and you just repeated that over and over again. Games have moved on since then, but its always nice to be reminded of how things were.