I'm surprised that no one talks about Synapse Toxic II. It's an excellent soft synth for very good price.
User Interface: Nice, clean and professional GUI. Everything is on one page - no switching between pages, tabs etc. I like the colors too :) Overall, that's the kind of design I really like.
Documentation: There is some documentation (chm format) which describes each Toxis module but actually I don't need it :)
Presets: There are about 600 presets divided into 10 Sound Banks. Most presets are really good - especially i like Pad SoundBank. You'll find bass, leads, pads - it's almost everything there...
Value For Money: Seriously, 49 $ for this monster is one of best deals ever. It's a killer synth capable of creating almost any sound - pads, leads, basses, arps. Judging by it's features, it should cost about 99 E...
Stability: Never creashed (at least not in Ableton Live which I use)
Things I like the most: * 6 (!!!) alias free Oscillators - each with 50 waveforms available * FM Matrix - great for pads and "electrical" sounds * gorgeous Sequencer with load / save sequencer patterns option (the best one i've ever seen ??) * powerful effects section (EQ, Distortion, two Chorus, Flanger, Delay units)