I found this game cheap in a second hand store, and after playing it wondered why I had never heard of it before!
Its bsic idea of combigning different parts of robots sounds famaliar, but it plays like something entirely different to anything you have ever played before, with three different views (underground, surface and air) combigning to make it a very strategic game.
As well as the massive robots of death you also get smaller tanks and stuff, like it C&C games, along with planes, diggers etc. There is also a promotion system like in RA2, so your robot crews can gain experience.
The storyline is great (read the other reviews, someone else commemted on it) and the three different sides are vastly different, with amazing upgrades on each, making you adopt a different stratefy for each.