Unification Wars (and GC, their other world) is the most well done game of that kind I ever saw.
They might well be #1 on the net, and it is surprising to see how many are ready to pay a little to get bonuses. Players love the developers and are even paying the guy's hospital bill
There are numbers here and there but
_everything_ is done to flatten the learning curve (the strong even helps the weak to grow), and it's sand-box style including even a stock-market.
I tried some other games like this to see how they were, but nothing close (and barely anyone was paying for more). And the community is really friendly with some messaging each other (attackers don't even send insults

), the opposite of some other places. There are even some *gasp* girls
Have fun and feel free to report here