I really enjoy this synth. It is easy to program and sounds great.
UI:Please note that the newest GUI is completely redesigned and looks great. Simple, clear, catchy. Doesn't hog the screen. Plus, I like sliders much more than knobs.
Sound:Wow. It just sounds great. Very thick sounds possible. Despite the name, this is not a 303 clone only. It is very flexible and capable of a wide range of basses, leads, and maybe even a pad for a creative type. The filters seem nice, as well.
Documentaion: Straightforward pics of synth with midi labels. No tutorials, but its awfully simple.
Presets:decent variety
Customer support: I have no idea, never having contacted them, but they do have an active forum on the site, primarily in Japanese.
Value for money: It is a great time to live in when synths that sound like this are free...
Stability: This leaves a bit to be desired. Occasionally, loading user presets hangs the machine, or loading a preset and then cycling through the OEM presets, the user preset vanishes and/or resets to the original. Still very usable though.

This plug is as deceptive as my other favourite buzzroom plug 'Octbuz' in its ability to produce a wide range of sounds / treatments that are not apparent without some investigation and time.
I have managed to coax a whole range of percussive , analogue , digital and reasonably natural sounds out of this synth.
I do have an issue around saved sounds in as much as on going back to them i have found that after several choices the synth refuses to respond , this may hopefully be worked around , but it does not make the instrument unusable - it might be an issue with my set up?
Getting back to the BB303i i could imagine along with OctBuz for fx treatment making a whole tune with it , as stated before i have managed to program bass drums , snares , hi-hats , all kinds of bass , drones , even a piano , electric guitar and varios sync fx!
What more could you ask for its FREE , sounds great and versatile.