I haven't listened to a lot of Nirvana, but from what I have heard I think their big hit, Teen Spirit, probably isn't one of their best songs, so they might suffer a little from that in the ears of casual listeners like myself. My take on them is that they had a good sound, and Cobain had an awesome voice, one of the best in-tune screams in rock history, but their song-writing wasn't of the very first rank. Certainly one of the best of the grunge scene, though.
But I'm generally not a huge fan of the whole grunge era. Not that it didn't produce a fair bit of very listenable music, but I think it was pretty over-rated. It sounded fresh at the time because that sort of basic, no-frills hard rock hadn't enjoyed widespread mainstream popularity for a quite few years previously. But it didn't really bring any new energy or ideas to pop music, just re-hashed stuff that had been done better years before. Unlike the punk era, which IMO really did bring something new to the pop scene.