Just completed this, wasn't expecting a great deal (got it in some THQ pack I bought on Steam a while back) but it turns out that it is actually a pretty good game. Graphically it looks decent, excellent framerate and is different enough from standard shooters (drones etc) to keep it interesting. If I'm honest I'd have to say that I enjoyed the SP more than Battlefield: Bad Company 2, the weapons just feel good (in terms of aiming) although maybe a little underpowered.
On the downside the missions are largely repetitive, basically you either have to control an area or plant a demo charge on something and blow it up for 90% of the objectives. The claim of "non-linear gameplay" is nothing short of outrageous; choosing which objective to go for first is about the extent of the freedom. It's also not the longest game, approx 5hrs gameplay time which although fairly short is certainly no less than the likes of Crysis Warhead or the Modern Warfare games - when the action is intense it is more forgivable.
All in all I would definitely recommend this game to FPS junkies in search of their next fix, maybe wait for it to crop up in steam deals again (has been £3.74 on it's own or in the THQ packs).