well must be a rarity, I really enjoy the d20 system. Not just DnD, I haven't actually ran a 3rd (or 3.5) edition og that game in nearly a year. Now BESM d20? well well, that game is so open that the players could drive a mac truck through it and not scratch the paint. If the GM lets them. DnD has taken much of the control away from the GM(DM), but BESM d20 allows the GM to control every single aspect of the game without being to intrusive. The same is with d20 Mecha. The main mecha should be designed by the GM, while little extras can be added by players with GM discression. The combat is not about the system, its about the quality GM running the game. It's up to him to paint a picture of massive mecha battling out duels among giant space ships.
Another thing I have come across is this: If you put stats to somthing, the players will find a way to kill/main/break it. Don't be such a stats hog and the game can be ran much like anime, whatever is more dramatic/silly/scary/whatever will work, and every one will have more fun