The sounds on the O.S. are really excellent. The fuzz is excellent, the filter is excellent, the only downside is that while the oscillator sounds excellent the tracking struggles to keep up, it can be very unpredictable even if you're used to applying all the tricks you need to make tracking pedals work their best.
If you use filters for a lot of different stuff it's worth getting one just for the programmable filter with ADSR, LFO, sample & hold, tap tempo etc. If you use fuzzes in conjunction with filters a lot then again it's probably going to be useful to you. If you want the waveforms then look elsewhere - it just doesn't track reliably enough.
The last time I emailed Chunk they were working on an improved approach to the tracking problem but it's been over 18 months since they put out a firmware update (well, there was the .10 update but that was just to fix some stability issues) so I wouldn't count on the tracking improving any time soon.
As for the interface, it's fiddly but not hard to understand if you read the manual. I don't have any trouble getting the sounds I want out of it.