I've never seen the Japanese series', unfortunately, though I've heard plenty about them.
As for Transformers, it begins and ends for me with G1, Beast Wars, and the comics. I loathed Beast Machines with a passion, was unimpressed with Robots In Disguise, and was once again repulsed when first Armada and now Energon hit the air (It says quite a bit when you find more consistent writing, better storytelling, better dialogue, and a more coherent continuity in G1, which is 20 years old, then in Armada. For cripes sake, at least G1 episodes maintained continuity within themselves. I've seen Armada episodes in which the events later in the episode contradicted what happened only 10 minutes earlier!). I feel like I've been out in the wilderness for the past 5 years, suffering through never-ending crap, waiting forlornly for SOMEBODY to put a good Transformers series on the air.

Fortunately the comics which I've picked up have been really good, but I never have the spare change to go out and buy them, and I'm completely unused to collecting comics after all these years, so I'm always behind. Oh well. Fortunately it appears that they're publishing TPB's, so I'll see about getting those, eventually. Especially The War Within. Can't wait to finish off that series.
But yeah, I've always figured D20 could cover all those situations, though I never really put any effort into doing any converting, myself. Nice to see someone has had some success. As for Mechamorphosis, while I hope that they'll have something similar to the various Masters, I doubt it. The gestalts, though, are what I'm really hoping for. Especially if there're rules which make forming into a gestalt advantages to a group of PC's, rather then leaving all but one of them out in the cold, having nothing to do while the gestalt battles it out with whomever.