Those first ten minutes were painful, as every line tried to be some great one liner and generally failed.
After that (about the time Michael Cera's character got introduced, and they were done setting up Juno as this super-quirky character) I grew to enjoy it quite a bit, though I never got into her best friend, where all the other characters worked for me.
Almost, anyway. How could a girl who digs Iggy & the Stooges, Patti Smith, Mott the Hoople and The Moldy Peaches etc not have heard any song by Sonic Youth?
Also, there was a girl who sat behind me, who audibly laughed three times in the cinema. Each time was in the previews, when someone in the shitty comedies they were advertising got hit in the head with something. Man, she loved that shit - each time she laughed was more uproarious than the last.