Those answers were right, but now it is 2013, so some update are needed.
Now majority run on Android Ice Cream Sandwich, some on Android Jelly Bean.
If you run into some tablets on 2.2, you know that they are long life stock. So don't hesitate to cut the price since you are sure that the seller is eager to get off with them.
And the price is $150 below. With $200, you get something better than new iPad.