I'll have to agree with those who mentioned DBSK/TVXQ...DBSK are very talented vocalists there's no doubt about that...but I also think their popularity is "insane", sure they're talented...but I think their fame overall is just too much.
I guess I can understand why the JE groups are mentioned a few times here too...like DBSK, these JE groups are famous & some of them are also talented to top their looks (don't think they're only popular due to their looks, because that's so NOT true!)...but the JE also has its share of antis as well...and naturally there's going to be some ppl who think these JE guys are over-rated.
I personally don't think KAT-TUN are over-rated though (as a whole)...obviously!

however, individually though, there are members in the group who I think are over-rated/under-rated. So, this is my analysis on the KAT-TUN members...
Over-rated:Akanishi Jin - he is the 1 member who comes to my mind first! I like him, but I think he's the most over-rated in the group...he's always highly praised by his fans for having the "best" vocals in the group & being the "best" performer...frankly, I beg to differ to some extent...I actually don't think his vocals are the "best" in the group really. If I was to pick another KT member who I feel can really level with Jin in terms of vocal skills, or even as much as surpassing Jin, then that member is
Junno! Junno's voice is strong & crisp, he's able to execute his songs & the group songs well every time & is able to maintain his vocals even as he moves & dances.
Sure Jin has a really good voice...but it's a bit weaker in comparison to Junno's & he sometimes has a bit of trouble holding a note for long...at times, you can even hear the tiredness in his voice. So I don't think he has the "best" vocals in the group...as for performance...well, I agree on that, Jin is really a 'natural' performer, he has this "WOW" factor that can really draw you in. But again, I don't think he's the "best" in the group. I have all their concert dvds & watching so many live performances of all the guys both together & solo, I realized that they're ALL fantastic performers & dancers and they're ALL able to bring their own unique qualities & styles to the stage.
Jin is arguably the most popular member in KT, along with Kame...but I personally feel his own popularity is just too much...like DBSK, I acknowledge Jin's talents & potentials...but still, I feel that the overwhelming amount of his fans over-rate him a bit too much...I don't think he's the "best" in the group.
Kamenashi Kazuya - and ironically, Kame's my 2nd fav in the group...don't get me wrong, I love him a lot...but again, I think he's a bit over-rated...because vocally, I'm slightly irritated that Kame is so much more appreciated for his vocals than Junno is. Although he can still sing well & is able to execute the songs, but Junno's voice is still stronger & better. And so are KOKI & Maru's vocals, they're both fantastic singers too...in terms of singing, I prefer these 3 guys' vocals, than Kame's.
Under-rated: ALL of KT's other 4 members!
TANAKA KOKI - my ichiban!

: In my opinion, he is MAJORLY under-rated both in the group & individually! I mean, he's a tremendous rapper & rap lyricist, great dancer & also a 'natural' performer! He's a fantastic singer too, his vocals are pleasant & often he really feels the music & sings the songs with feeling to compliment the mood of the songs. He sings with personality, raps with personality & he often let his body connect to the music & he starts to move & dance naturally. Along with his bandmates, he's also a versatile singer...the group sings jpop, jrock, ballads, RnB etc...but Koki-kun himself also sings hard rock & even attempted (briefly) at opera! Which, I was very impressed because he was amazingly good at it!

KOKI is also a promising actor, obviously better than Jin in that area.
In terms of overall talents & potentials, KOKI definitely ranks up there with Jin! But he doesn't have nearly as many fans as Jin & Kame does. So yes, I definitely think KOKI is under-rated in KT!
Nakamaru Yuichi - what can I say? Maru-kun is a good singer too, really good dancer & performer as well...and his human beatboxing skills, what can I say, outstanding!

but like KOKI, he is under-rated, he also doesn't have that many fans compared to Jin & Kame.
Ueda Tatsuya - I used to be quite critical of Ueda's vocals because it's weaker than the others & often when you listen to the studio versions of the group songs, Ueda's vocals just pales in comparison & sounds more like he's whispering. But after listening to his solos & watching his solo performances, I realized that this guy DOES actually have a good voice! Just that his vocals are still a bit weaker than the others, so when the group sings & performs together, the others tend to over-power Ueda's vocals...but in his solos, you can really notice the true potentials & skills of Ueda's vocals. He is also a fantastic pianist & guitarist, and very gifted lyricist.
Taguchi Junnosuke - and finally Junno...like I have mentioned before, his vocals are strong & crisp, perhaps the best vocalist in the group, his voice is un-doubtfully lead vocal material! He is also a fantastic performer.
Overall, as much as I adore KT as a whole...I have to admit that the excessive over-rating of Jin & Kame often irritates me...I still love Kame-chan of course & like Jin, just that I find it annoying when both their fans over-rate them too much.
I think what the problem is with some of the JE groups (particularly KT & NewS) is that...the Jimusho tends to promote particular members from both groups more than the other members, sometimes they promote those certain members a bit too much, they gain so much attention & popularity, that the other members are virtually left in the dark.
This was very much the case in KT...since the very beginning, both Kame & Jin have been promoted & exposed the most & have remained the most popular...and unfortunately, often the group are being judged by others just based on Kame & Jin alone, which is very bias in my opinion. If ppl observe the other 4 members closely, watch their solos, they'll discover the other 4 are talented too, have their own uniqueness & traits also. Some of them are just as talented or perhaps even more talented than Kame & Jin are!
Although I'm glad that KOKI, Maru, Junno & Ueda are all gradually becoming more popular now with more ppl appreciating them & they're slowly getting promoted more by the Jimusho...but I still don't think it's enough really...I believe they should by now have the same kind of popularity or at least be given almost the same amount of credibility & fame that both Kame & Jin have...sadly, that's not the case