A great and funny game.
Though this is free to play you have NO possibility to buy ingame advantages.
You can use your steam wallet's cash to buy great looking armor and weapons. This stuff has no influence on your stats.
You can also buy boosters but they are totally worthless and won't influence much. :-) So don't worry.
This game is more about tactic than about going after kills. The more of the enemy bots you kill the faster you level up. 2nd is you shouldn't get killed. Third is: Destroy the enemy.
If you are interested in this game, just try it. It is free anyway. Just one important thing:DO NOT START SUPER CROSSFIRE GAME MODE
Because of the small playerbase it will take forever to get a match at Super Crossfire. Just play TURBOCROSS. You will find matches even faster. Sometimes I have to exit the lobby and restart so I get a match after about 2 minutes. But that's really the worst case scenario.
There are even some nice guides.