First off, it's definitely unique amongst Dave's work. It's got less gain than most (tho' it's not short on gain at all),
it's got a little more treble bite (tho' never shrill) and it's really good at blending in with your existing tone.
Many of us buy fuzzes 'cause they make this rad, wicked sound, but to be honest, that sound is often a real game-changer.
Plug into Dave's Fuzzrong or maybe the Meathead and you get this glorious but transforming fuzz experience.
The FS-673 doesn't do that. Instead, I think it takes your base tone and seamlessly blends itself into what you've already got going on.
I mean there's no doubt you stepped on a pedal and there's no doubt that pedal isn't packed with character and depth of it's own,
but it does so in a way that doesn't upset the balance of things.
The tone is slightly compressed and bears subtle hints of the MKI/1965 kind of grit, but without the uncontrolled element.
It''s got the treble bite of the 1965 too, but with a beefy lower-mid content that preserves your original tone.
What also sets it apart is that the gain knob works anywhere you set it on the dial.
It gets thicker as you turn it up, but it still has plenty of beef, crunch and consistency at 12 o'clock, and you can clearly hear the differences.
A lot of the more primitive fuzzes (that we all love) don't always offer much with the gain set low that you can use in a 'normal' way.
This pedal is refined for sure and shows that Dave either got really lucky or put some time into making something different.
Too bad it's unobtanium because I think (short of it's size) it's one of the most gig-friendly pedals Dave has ever made.
OK, I could go on for a lot longer, but I gotta' get out and enjoy the sun ... Spring has finally arrived here!
It's really is a fuzz pedal at heart but reacts and works much like a really great overdrive would. It cleans up very well and the fuzz knob really tightens up the character of the pedal when turned down. Like Cubba said, the range of the controls is entirely useful across the full range and also quite subtle. I find it to be a somewhat darker pedal, in a good way, much like a nice tubescreamer. I initially was playing my strat into it through my gibson scout amp, which is very clean...when I switched to my friends vintage gibson es-330 (which has hotter P-90 pickups than the strat) into my little supro super (which basically has no headroom) the FZ really really came to life!! 
It really works best in this application IMO, hotter pickups (well, still single coils in my case) into an amp that's already cooking a bit, really wonderful sound. The filter control is subtle and totally reminded me of the filter control on the drag n fly in that it adds a bit of gain on the bassier end and it's quite subtle although again, completely useable across its entire range. Basically this is a really great build, as if anyone needed convincing!! 
