As much as I like(d) Last.fm, I'm finding MOG to be a better music listening/sharing service. For one thing, I found that the Last.fm "scrobbler" was eating a bunch of processor and memory on my PowerMac G4. After switching to MOG, I found that it was MUCH less obtrusive. Their "MOGifier" is a preference pane in your System Prefs, and uses about half as much memory as the Last.fm scrobbler and it's much less processor-intensive.
Beyond that, MOG is related to Gracenote, the same service that identifies your CD's when you put them in your computer to play them (or rip them).
Give them both a try and see which you like better.