The game is currently $19.99 on Gamefly. Since I have a 10% discount for being a member for so long, that dropped it down to $17.99, plus every three months I receive a $5.00 coupon, bringing it further down to $12.99. I bought it.
I've completed the first two missions so far and have boosted challenges a couple of times. The game is eh; your typical FPS / get to the end of the level gameplay. Nothing special. I'm not even paying attention to what's going on; just playing it to play it. I wish the waypoints were clearer (although the motion tracker does show them) as there were a couple of times I had no idea where to go or where my A.I. teammate was. I haven't experienced any glitches except xenos that crawl into and out of a solid wall and a couple of places where I was shooting an enemy and it wasn't hitting them.
For $13, it's worth me and my friend to get a good amount of gamerscore in, but I wouldn't have paid over $15.