Custom Robo is plenty of fun, provided you have other people to play it with.
Just like Gotcha Force, the single player adventure is fair to middling, but it's just an excuse to make you earn things (parts, etc) for multiplayer.
Sure, it's not Super Smash Brothers Melee, but then, how many games actually rise to that level?
I'd not suggest it for people at full price, but at 15? You'll get a good couple of hours play out of it just to earn your upgrade parts, and then a couple more tinkering around with it in multiplayer.