I always liked Nirvana.
I can understand people saying they are a bit overrated though, but only because they only made three albums (4 with the unplugged) and I personally think three albums is not "enough" to really show all the potential of creativity of a band.
Some would said that plenty of bands should have stopped at three albums... but I don't agree. I like my favorit artists to have a long and complete discography, with ups and downs, to really show what they are capable of on the long run, and not just few good singles on two or three albums.
But in the case of Nirvana it's a bit different. They were the band (with some others, but the history will keep Nirvana only) that rediffined rock music in the late 80's early 90's.
With only two albums (Bleach and Nevermind), they threw a big "fuck you" to all the guitar heroes who really started to show off a bit too much at that time, IMO. They showed everyone that with few guitar skills, but some good riffs and plenty of energy (but also a kind of "I don't care / punk" attitude that will kill them in the end) you could make some awesome songs and concerts. In Utero and the Unplugged started a bit to show the big potential of Nirvana, but alas, we'll never know. I am sure Kurt had many other albums left in him. He just asked the wrong muses.
In some way Nirvana is what Rap music also was at that time: they both were music that people could say "Hey I can also do that! Awesome, let start a band! It's not difficult". Really kind of like the Sex Pistols and the Punk movement more than a decade before. Only this time it stayed a bit longuer.
Music is often a reaction movement: too much 70's progressive rock = Punk = guitare heroes = Grunge = Rap Metal = return of 60's rock etc...
Of course being a great band is not that easy, but they showed the way to many future musicians who maybe would have been to afraid to grab a guitare and start playing because they did not think they had the skills.
I will end with a quote of Kurt Cobain (or what I remember of that quote):
"Even if you only know two guitare chords: you have to play them well."