I suppose they are a good gateway band to discover more legit punk based bands. Were once good live in the early "Ashkanaz" days. Cons: There picture is in the dictionary next to the word "sellout". Have gone from pop punk to corporate pop.
Had a great career and catalog of material from their beginning to the early '00s. Good lyrics, catchy music. They were also my first favorite band. :D Cons: They have gotten comically political. Too angsty. Too many 10 year old fans. They've unfortunately made a joke of themselves.
Nirvana was tapped into the soul of the era (zeitgeist) and changed the landscape of music overnight. Cons: The perception of their musical "originality" is a bit overblown in the historical context of R.E.M, The Pixies, Dinosaur Jr. and other such bands that Nirvana heavily emulated.