For me they were not overrated. In fact, they were THE premier metal band of the 90's and one of the top metal bands of all time.
Compared to other metal bands at the time, Pantera were in another realm. They had the best combination of heavy, power, and groove. They carved their own niche. Other metal bands didn't have that or they didn't pull it off (heavy/groove) as well. In the 90's you had old school Priest, Maiden, Ozzy, etc. still doing their thing. The thrash bands from the 80's either started changing (Metallica, Megadeth) and went not so thrashy or didn't evolve much at all (Slayer, Anthrax, Testament). Metal began to expand with sub-genres too but Pantera stood out above the rest. It was infectious.
I saw them 4 times live. They usually took out good bands that would open for them (White Zombie, Machine Head, to name a couple) and Pantera was just that much better than them live. They basically headlined Ozzfest in 2000 and while I didn't go, many say that the venues would half-empty out after Pantera was done before Ozzy took the stage.
You had the character & incredible guitar playing of Dime, solid drumming of Vinnie Paul, and people don't realize how musical Rex Brown is on the bass. Listen to Pantera songs and concentrate on the bass lines. He's not just playing along with the riff. He plays around the riff.
Plus, their material and influence stands the test of time. It's not like Limp Bizkit where you hear the music and think "90's nu-metal." Metal fans still seem to hold Pantera in high regard.