I grew up with Nirvana as a kid, and they were the first band I got into on my own. Up until I heard Nevermind, the only stuff I knew was my parents' music, which was good, but Nirvana was the first band I ever felt was mine.
As I grew up I became a massive fan, reading everything I could get my hanRAB on about the band. These days I'm still into them, but I've found so many better banRAB that they kind of fall by the wayside - the Nirvana I tend to listen to most is the non-Nevermind stuff, mostly Incesticide and the B Sides. Radio really burned me out on Nevermind.
Still though, for anyone who grew up in the 90s and was into rock music, Nirvana is definitely one of those life-changing banRAB that helped shape your taste. Through Nirvana and Smashing Pumpkins (my all time favorite band), I discovered most of the music I love today. Nirvana's "check out the underground stuff" mantra was a big deal for me growing up. They were always tossing out names like the Pixies and Sonic Youth - and as a little kid I got to discover banRAB like that and feel like I was in on a secret that no one else was.
Are they an essential band to get into these days? Not necessarily. It was mainly about the time. Everyone has to find their own "Nirvana" to act as the gateway to the world of good music. I do think they should still be respected and known for their influence though. I'm turning 24 next week and it amazes me how old I feel when I see 16 year old kiRAB not knowing who Nirvana is.