In this game you play as robots who are totally customizeable, hence the name. You can equip various weapons, back mounted pods, parts to boost speed or jumping height, and so on. You select from various robot frames which have different levels of agility. You unlock those as you play and can also change the color of your robot as well.
The game's story mode isn't that interesting but its mandatory to unlock new robots and parts to improve your strategy and keep up with the steadily rising difficulty. The story is boring and corny so your only motivation to continue playing is to unlock new items to show off your skills in multiplayer versus or tag team brawls.
In terms of controls, you really won't have much difficulty adapting to this game quickly. Its really a lot of button mashing on A to shoot and dashing around in enclosed arenas which aren't really awe-inspiring - they're very basic looking with elemental themes for the most part. The battle scenes can be very chaotic and while the graphics are great, your robots are miniature and you can't see as much detail during a match. It supports up to 4 players which is pretty cool if you have siblings or friends over. Since its a Gamecube game, its also compatible with Nintendo Wii so that's always a plus too.
The multiplayer aspect really makes this game more appealing because solo play gets dull after a while. Rather than split screen for multiplayer, everything goes on in 1 screen. I think the bottom line is that this is more like a fast paced shoot out - who can dodge and deliver the most damage. Its gets very repetitive.
Right now the game is going for around $10 so for that price it sounds like a good bargain, however if you own a DS, there is a new Custom Robo with online play available so you may want to look into that since its portable and more likely to maintain your interest.