I liked it well enough to watch it a second time, and then I really appreciated it. The acting by all is truly very good. I hadn't even noticed how good the boy is the first time around.

Juno's annoying lines in the beginning are signs of the protective ironic-teen carapace of the character -- which falls away as she faces life.

She starts out treating everything with smart-aleck jokes, then she changes. Watch the actor's face register these changes.

It's not a sign of a flawed movie that Juno starts out being annoying. The movie didn't started out bad and get better 20 minutes in. Rather, the character progressively grew.

You might as well say that Jane Austen's "Emma" starts out bad (because the character is annoying) but the book gets better. It's called character development.