I am playing it for 3 days. I wasn't really impressed at first but the more I play, the better it is. I really love the numerous small things that got added to OA like the kickback, scope adjustments, head movement, FLIR of course and many more things that you may not notice at first. Haven't finished the campaign yet and I'm suffering from some annoying bugs. The singleplayer missions are awsome so far. Played Laser show and One bullet is enough and they were really atmospheric - the only problem is that they are too short.
I was also surprised how cheap I got it. I had preordered for 25€ + 4€ postage, but then I found it in one shop for 22€, so I cancelled the preorder.