I don't remember buying a "soundtrack album" since Pretty in Pink, until recently that is, and I've bought two in the last couple months. The music opens up Juno and I caught myself thinking thinking that it's going to be a loooonnng movie if we have to listen to this low rent folk for 2 hours, but even that first tune (All I Want Is You by Barry Louis Polisar) quickly grows on you as you realize that it gloves the movie.
Back to the first soundtrack, which was August Rush, I got it for my wife for Christmas. (I wrote about the film here.) Van Morrison's "Moondance" is the classic here but it's the interplay of the music and the world and the world of the film that makes this one special. It's rich and symphonic and Irish rock and experimental; ethereal then cathartic.
Juno's got The Kinks, Mott the Hoople and The Velvet Underground providing classics (and Sonic Youth's cover of The Carpenters' "Superstar" which I hadn't heard before and it's pretty devo). But it's The Moldy Peaches and Kimya Dawson that rule the film and set the tone. The simple arrangements and acoustics sucked me in to the characters. Not that I identify with Juno herself much, at least not with her predicament (Uh, teen, pregnant). I didn't feel any parallels with Bleeker either (Hmm. What'd'ya think, Cindy?). Maybe high school was just too long ago. I did empathize with Juno's dad (J.K. Simmons); and with the Would-Be-Adoptive-Father-Wishes-He-Still-Rock-n-Rolled (Jason Bateman).
And the Juno soundtrack has Liner Notes! They're written by the Director, Jason Reitman and provide nice insights, just like Liner Notes of old!
Come to think of it, my son did buy the soundtrack to O Brother, Where Art Thou? when it was fresh (2000) and these are in that same vein.
(I only see one soundtrack album on David Rawson's top album lists (1996's Trainspotting came it at #72 and he usually only takes the top 40), but there are more individual songs scattered among his top 100 songs lists so maybe there's something here to his liking....)