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I have been playing this for a couple of months now, albeit with only limited gaming time, but am still enjoying it.
I made a one-time payment for each of the three civs I own, and feel like I got a relatively good deal. Most of the civs go on sale at some point, so you don't need to pay the full $20/each unless you're impatient (a lot like League of Legends; impatience is expensive, if you're willing to wait you can get some good deals). I've paid $50/total for civilizations, a bit more for pure vanity items, and have certainly gotten as much gameplay, if not more, as I would have out of a "standard" RTS, if such a thing exists any more. It's more like an a la carte game than a subscription, as everything you buy permanently adds significant content.
The pace of new content release has been slow, but it has been coming. Competitive Mode is coming soon and should address concerns over PvP; I am not huge on PvP, but I know a lot of players are, and that should help boost the player base. The big thing at this point is Skirmish Mode, basically a flexible PvE mode, which has been pushed back to December, unfortunately; but once that is in the game it should help add some longevity.
My main concern is that MS will not see the money they had hoped to see come out of this and will pull the plug. I would like to continue playing this for some time, as I don't see many games on the horizon that appeal to me. That would be too bad if they shut the servers down, but I wouldn't be surprised; there just aren't all that many players, even after all the fixes to the many problems it had at launch. The attitude on their forums is much more positive, but are there new players coming on board? At any rate, it's not the players' fault, MS really did a terrible job on the marketing and in launching it too soon - even a couple of months would have made a huge difference. Not sure how long this will be around, but I'm still here.