I guess it depends on what you mean by overrated. I could see throwing Frenzied Berserker on the list for CO builds, but if you're looking at non-CO players, there are other PrCs that they jump all over without realizing how terrible they are. A lot of them are Core:
Assassin - people seem to think that you need to wait until you're sixth level and write "assassin" on your character sheet to be able to kill people in a sneaky manner or something. Adding insult to injury, the signature ability of the class pretty much sucks. Play a rogue. Or a caster.
Dwarven Defender - it seems that a lot of dwarf fighter-type builds I see end up with this PrC. It's like they see "dwarf" written on their character sheet, and that there exists a class in the DMG called "dwarven defender" and they think these two things go together like Kahlua and cream. Pfff. More like skittles and beer! Oddly enough, I don't see too many elf = arcane archer builds. Maybe it's because it's not called "Elven Archer". I don't know.
Blackguard - okay, this concept is cool, but the PrC sucks. Just write "evil" on your character sheet and play a real class.
Duelist - actually, I don't see too many of these builds posted, but in a core-only game, some people get the idea that this is what you have to do if you want to be a swashbuckler. Cross off "fighter" and "duelist" off your character sheet and just keep "rogue" and call it a day. You have all the same flavor, and you only suck half as much.