Fabfilter One and Saturn are the only Fabfilter products I own. I gladly parted with the asking price for Saturn and am forever grateful for this transation, as it's the gift that keeps giving! Fabfilter One was I think Fab's first attempt at a commercial synthesizer, and they started out with the KISS methodology (Keep It Simple, Stupid!)
The sound quality of both the oscillator and the filter is exceptional, and I believe it was one of the first VST synths to really deliver a convincing emulation of an analog filter. I picked it up second hand because I saw it in the market place and can't resist a bargain. Would I have paid the full price for it? In my current situation, probably not, as I have the "analog" side of things pretty much covered, but now that I have it, I enjoy it for simple sounds, that filter, and it's overall character.
Around the 50 pound mark is quite steep for a one oscillator synth in all honesty. Had there been two oscillators, or some more sound shaping options, this wouldn't be an issue, but the reality is that while this was on the cutting edge of sound quality many years ago and well worth the asking price, it hasn't aged well in terms of bang for buck. Luckily, the sound quality it-self is still sounding pretty flawless, so this whole point is a matter of opinion entirely. YMMV.
The oscillator is simple but unique. The triangle and square wave have a considerable amount of even harmonics, which means you can't really make "hollow" sounds easily. The triangle's waveform is more of a distorted sine, but it serves it's purpose well, and I'm a fan of this purely because it's different, with a similar bandwidth as a normal triangle. Because the square wave sounds more like a saw and square mixture waveform, the sweeping PWM effect is a little bit less dramatic, with less cancellation at the nulls than is the norm, but it's still a recongizable PWM sound. The plus side is that those extra harmonics, again, make One stand out from the crowd.
One pet hate of mine is when a PWM implimentation doesn't let you specify a modulation *between* two pulsewidths, but instead assumes you want to morph from a square into whatever pulsewidth you've selected. Setting a narrow pulse and then modulating the PW a bit with an LFO or envelope is out of the question, though thankfully the envelope is inverable via a switch, which opens up a few more options.
Now onto that filter. This is really the instrument's saving grace. Despite being a single-mode, single-slope resonant lowpass (often compared sonically with Korg's MS-20 filter), it's very powerful in it's ability to transform sounds. Not only is there a complete lack of digital artifacts such as stepping and roughness, but the filter responds to high resonance levels not with an ear-peircing screach, but instead with a complicated overtone structure, which almost brings it into FM and ring-modulation territory at times.
I even managed to create a detuned effect by setting the filter to resonate close to, but not exactly on a harmonic of the oscillator. All of these effects are given a firm footing by the perfect and variable key tracking you can assign, allowing you to create some complex and interesting waveforms by using the filter to extend the harmonic structure of the triangle wave.
The GUI is spot on and responds to mouse activity perfectly, and the portamento is very musical sounding. However, portamento doesn't seem to affect the filter, which is a slight disappointment given that the filter is such a big part of the musicality of this instrument.
Overall, the limited features of One and the great sound quality combine to create a synth that takes you back to the "good old days", where creativity with knobs and switdhes was the order of the day. It can create a fair amount of tones for a 1-osc synth, but I feel it falls just short of it's current price point in today's market.