Ezra Klein likes Skim. I can see why. I downloaded and installed the compact open source application, and even at version 0.51 it's sweet.
I'm an Acrobat guru -- but I'm not a fan. Adobe is not a great software house on XP, and they're truly weak on OS X. They ignore basic OS rules (such as not running as an admin user) and their application update process is profoundly broken. Let's not talk about their browser integration ...
So I know Acrobat Pro extremely well, including the esoteric bits, but I've avoided both the Pro app and the Reader on OS X. I use Preview, a suprisingly powerful application that's a bit short of PDF oomph. Skim adds real notes (not the weird image overlays Preview sort-of provides) and some nice PDF browsing capabilities -- and we're only at 0.51.
Skim looks very promising.