I just started to play this a week ago. It's addicting. I played it enuf to actually invest ~$50 but then I wasted the gold on crap (prem tanks and finding out they could not be researched to get better updates and then selling them) and doing stupid things that wasted the gold since I didn't know how the gold vs exp worked. Overall it's great/fun free for play game.
ADD: the graphics and effects are actually prettyy good looking. Only uses one GPU tho.