We joined these sites through a freebie that I shared here and the kids have been trying it out ever since. I recommend enabling the social features because that was a main selling point for my stubborn youngest. Her first reaction to the site was, "You tried to trick me! It's just dumb learning games!" When I called my son in to set up his account, she sneaked in and saw how fun it was and came crawling back for her turn. The parenting site lets me assign specific activities for them to complete for game currency (scoops for her and gold for him) that they can use to buy items for their house, purchase eggs to hatch pets, and buy items to take care of their pets. They can also make things and sell them in a store or babysit other members pets for pay. The store aspect is neat because it teaches them about setting prices and making profits without the stickiness of a lemonade stand, which would be futile in our somewhat isolated neighborhood.
The learning aspect of the site is fun because kids are rewarded for completing activities in all subjects and then they can enjoy fun time as well. I assigned my oldest some reducing fractions problems and he did it without complaining. Any questions that the kids miss are repeated as review at the end of the level, so it's also a teaching tool. We haven't covered countries much yet, so when they played that game, a lot of it was guessing and then they'd see the location and repeat it at the end. So it's not just review but also a chance to gain new information. It is such a good site that I am considering whether to continue it as part of their curriculum instead of purchasing supplemental books, but I will decide at the end of summer whether to have them go back to webkinz to get their pet fix for free and keep up with the less detailed learning game sites.