Full disclosure: I am pursuing a PhD in education, studying video games. I have three children, ages 11, 15, and 16, all sophisticated gamers.
This is a good, interactive math worksheet with immediate feedback and a game-like reward system, but my children tell me it is not a game. Users will become familiar with algebraic manipulation rules, but not with the mathematical reasons behind them. The inability to choose a suitable starting point combined with the forced linear progression through the chapters and levels could prove boring for some, and too challenging for others. There is little replay value for 5th and 6th grade students. A teacher could really build off this if a classroom full of students played it.
For something more sophisticated (and expensive) try Dreambox (dreambox.com). My children did not think it was much of a game, either, but it does address the issues of finding a student’s correct starting level, and providing reasons for what the player is doing.