I think this thread covers all sorts of opinions about Nirvana. Well here's my 3 cents (yea Canadian dollar, baby)
Sound: Nirvana took 80s Punk, the Meat Puppets and folk sound, and mixed them together, with an attitude.
Lyrics: Smells Like Teen Spirit sucks balls, and it sucks balls on purpose. I think it was meant to sound like crap just to piss people off. Most of their lyrics were quite sincere.. Heart Shaped Box is a lil thought provoking, for example.
Leaders/Gods of Grunge: pfft.. HELL no! They were last Seattle band to join in. However, they marketed their material well enough, that the whole world listened to them. Mudhoney, Screaming Trees, Melvins (the fallen bridge between Punk and Grunge), Soundgarden, Temple of the Dog, Alice in Chains, Pearl Jam, and Mad Season were the real grunge, and will forever be.
Nirvana brought it (differently) to the mainstream.
Side note: Rollins is balls-to-the-wall badass!