Got this game as an import, it's quite good, much more than I'd have expected from a simple series game. It's kinda like driving the Metal Slug in the game of the same name, you have a tank with a cannon and a rotatable machinegun (both can be replaced with other kinds of weapons if you find the right parts), hop from platform to platform, shoot other tanks (including real tanks from different eras all on the same map...), etc. The Custom part means that you can replace parts on your tank with different ones, from what I see that's turret, sidearm, body and threads. There's many parts that are just different powerlevels of the same thing but still some variety, including indirect fire, homing missiles, spread guns and so on. You get multiple base tanks that each have their own compatibilities and part lists and you can level them up (not too much of an effect from what I've seen). It gets hard pretty quickly but extra lives are really plentiful.
Seems to have multicard two player coop but I can't test that.