I didn't particularly like Nirvana back when it happened (I thought they were horribly overrated, which I still think they were back then), but some of their stuff has grown on me in later years. They did some pretty good pop songs, and I liked the
Unplugged record - generally, I think they were better at doing acoustic pop tunes than noisy hard rock. (I love noisy hard rock, but there's a million bands that do it better than Nirvana. :p )
And generally, some (most?) of my favourites of "their" songs are in fact covers. I love "Molly's Lips" (just the same two chords all through the song - brilliant!

), "Son of a Gun", "Jesus Doesn't Want Me For a Sunbeam" and "Where Did You Sleep Last Night".
Theo