Nirvana was simply a back-to-basics rock band that came along at the right time. They were pretty popular when Cobain was alive, but nowhere near as popular as they became after the loser killed himself. I believe that had he lived, they would be rated right now in the same class as maybe Pearl Jam.(which isn't too shabby, popularity-wise)
I liked Nirvana when they came out, but let's face it: rock is all about the live show, and live, they sounded like crap. Cobain sounded like a dying cow, and their playing was very sloppy. Not the stuff of legends. (just listen to the unplugged album for proof)
He died at the height of Nirvana's popularity, and that is the sole reason why Nirvana has the status they have now.
Keep in mind, I did and still do like Nirvana, but face it, they were not all that.