Well, as far as I'm concerned, they're not overrated, since here, that's Holland, which is where I am, that means residing, in this country, this place, Thursday hasn't even been rated yet, let alone overrated. That, of course, means they're not underrated either, but they deserve way more attention than they get over here. I personally find the music utterly brilliant, just as the vocals. I don't really bother with the lyrics though, which I never do in general. They aren't really special anyways; what's up with a line like 'Someone call the headnurse / she is coming to the capitol' anyway? Something like this doesn't really fit the music, but well. The thing I appreciate about them is how they aren't really whiney like a lot of the newer bands.
We mustn't forget the bands that influenced them though. I think a lot of dues should be paid to bands such as Embrace (Ian McKaye), Texas Is The Reason, Fugazi, Snapcase, At The Drive-In, and even Deftones. They all influenced the sound Thursday produces. Yet they have some kind of individuality to them I can't really put a finger on, but then again, it's one of their strong points. I really think they are something different.
By the way, what other 'screamo' bands are you implying? (just curious)
On an interesting sidenote, the name 'screamo' has been used to label grindcore-like bands as well, like Skit System. Quite something different than Thursday, I must say!
Oh, have you ever heard something by Envy (especially their newer material)? Brilliant band. W00t.