I loved it, if you didn't that's your bad luck
I like to go with the bad news first. It's a short game. It has 3 guns. It doesn't do anything innovative or different to distinguish it from other FPS games. I've thought long and hard, and those are my only complaints.
The good, the hokey but fun story is a continuation of the original Sin (which comes with the game) and I enjoyed following it. Many of the reviews mention the large breast size in a negative tone, I happen to like them. The action is very easily tuned to your abilities, and the game keeps track of all manner of stats, from your hit ratios with the various wepons to the number of footsteps taken, it seems every keystroke and its results are being tracked in one way or another. While I do like my games to be inventive and surprising, and Sin falls flat on both fronts, it more than makes up for it with solid gameplay, and fun levels that make good use of the Source engine, for an appealing presentation.