I just got an SD 805 today, played with it for while and compared it to the Maxon OD808, which is the TS808 circuit.
It's pretty far from being a Tube Screamer knock-off. The pedal is louder, brighter, cleaner and less compressed overall, the clipping isn't quite as smooth, and is focused in a higher frequency range. I can get the SD 805 to sound closest to the Maxon with the volume at 11 o'clock, gain at 1 o'clock, and all three EQ's at 10 o'clock. It's gain range is less, but it's volume range is much greater, so it's got more "volume booster" appeal than the Maxon. I'd say in sum, it sounds a lot more modern than the Maxon.
The bass control is most useful, it allows for a full, low end rumble. It's effect is centered at 90kHz according to the booklet, so the bass remains pretty tight, even when dimed.
If anyone ever hated Tube Screamers for the lack of low end, this goes far beyond remedy; it can potentially drown out the bass guitar. It's very well positioned.
The mids control is decent too, it's in the "WAAAAA" range, centered at 750kHz, according to the booklet. Turning this down creates a scooped Tube Screamer, very heavy tone. It really is nice hearing a more mild overdrive in a scooped context, not bright or harsh like so many "transparent" overdrives with flat mids. This thing pisses all over the Full Drive II. Turning the mids up gives a ratty overdrive tone, but this is not it's strong suit.
The treble knob is the weak link. It's not even remotely a presence control, which is a shame since there is a lot of clipping action in the upper frequency range that could be accentuated with such a dial. According to the booklet, it's frequency notch is 2.12kHz, which is a nasally "WEEEEEEEEE" sound, more upper-mids than treble. Turning it down darkens and weakens the overall output, but turning it up creates an irritating, piercing tone, so it's more or less an immobile knob, only sounding passable between 9 and 11 o'clock. I'd like to alter the frequency range of that knob, if possible.
If you like Tube Screamers, you have to get this pedal for bass knob alone.