well, they never released their best stuff, not until the recent box at least, and even then there's lots of other really good stuff that's shelved. They helped end shitty generic hair metal and basically did for the corporate rock scene what flipper, the adolescents, tsol and the dead kennedys did for punk rock.
Nirvana isn't really my thing... yes, I will admit that Kurt Cobain could write a good song... but he could not play a good song... his guitar playing was horrible and he couldn't sing to save his life... but I guess that's what grunge is all about... but there are so many grunge bands out there that are better then Nirvana... like Soundgarden or Pearl Jam... and I guess Alice In Chains is considered grunge sometimes... and they're good as well.