Yeah both the DragonForce guys are absolutely putridly wretched guitarists, although Herman is a little bit better. This doesn't relate to their guitar "skills", but I saw them both hanging out at NAMM, with a couple groupie looking chicks hanging off them, and Sam was being all loud and giggly like a drunk 16 year old kid (holding a cup of MGD Light or something)...it was amusing because it seemed like they were trying to get attention, but no one really cared they were there! NAMM of course is primarily a gathering of musicians, and it seemed pretty obvious how actual musicians felt about them. I think 99% of the people out there who would say that those two guys are good at the guitar, are people who don't play instruments and are really in no position whatsoever to make judgments on musicians...the same crowd of people who have bought into their band and have made their band successful.
I want to stick up for Adam D. a bit...I agree that no one ever really heralds him as being super outstanding, so it's hard to say that he's overrated. Beyond that, I actually think Adam is a super tight player with good tone in his hands...you can really tell that he "feels" the guitar, which to me makes him very respectable and a cut above many other "established" non-virtuoso type guitarists.
I think Robert Trujillo is a pretty overrated bass player. All the clips I've seen of him rocking out or "soloing" with Metallica have been very, very weak. If I've overlooked some redeeming qualities in his playing though, I won't mind being corrected!