I've used MoodLogic for several months now, and I must say that it has revolutionized my music experience. In terms of generating playlists, it leaves little to be desired. Using the song data gathered from many sources, MoodLogic attempts to find songs that share similar characteristics such as mood, tempo, release date, and genre.
From what I have observed, the song data used by MoodLogic has been fairly accurate. It identified about 86% of my songs by the song signatures it generated, out of which I have only found about 3 that were identified incorrectly. As their database grows, I activate more of my song library, thereby increasing the number of songs MoodLogic can choose from when organizing new mixes.
The creation of mixes is one of my favorite feature, whereby I can select a genre, artist, or song title, and create a playlist of songs that are much like it. Here is an example: late at night when I'm about to lie down to sleep, I can click on the genre Easy Listening, then click the green Mix button, then the button labeled Relax. Viola! A playlist with relaxing songs that "sound like" Easy Listening is generated. The same kind of thing can be done with individual songs and artists: select Loser by 3 Doors Down, click "Mix," and a playlist of similar songs is presented. (In America by Creed, Wasting My Time by Default, and Good Times Gone by Nickelback are among the songs returned.)