I wrote a pretty great review of Sleevenote for iOS for The Industry, even if I do say so myself.
In short, this is an app for music lovers who really care deeply about their collection. The interface is noteworthy: with a completely black background and no UI chrome at all, the artwork of your albums becomes the interface. I'm a big fan.
(If you read Chasing Perfection because you're an audiophile like myself, be sure to read on to find out how I use Sleevenote on my iOS devices. I use it with iTunes Match in a pretty clever way.)