The new DLC adds in 2 full campaigns, 7 unique maps, a unique set of historical vehicles, historical arsenal of weapons and plenty more. It's full on content and the price seems pretty reasonable.
I've played the game on and off for some time now and it's damn fun to play. It has a reasonably active community too, but as far as top-down tactical shooters go it might be one of the best available on Linux. Being able to drive vehicles, pick from various weapons and run across huge maps taking down enemies is damn fun.
I still remember how hilarious it was getting in a jeep, driving like a maniac across a bridge as enemy players were diving out of the way, it was glorious.
For people with multiple monitors, it does have one annoying issue - it will give all your other monitors a black screen. Hopefully that will see a fix sometime, but it's not a major issue and can be worked around.