Here's the deal. This thing sounds pretty good. And it's cheap. And it's built like a tank.
Now I wouldn't trade a Cali or Smoothie or Empress for example for an Optostomp. But I would consider one as a backup. It seems pretty transparent and isn’t noisy. It adds a nice little extra something. Turning the compression knob up past 3:00 gets pretty squishy but it is nice and transparent before that. There is not a lot of extra gain on hand.
The one I tested was sent to me by a Talkbass.com member and is the early yellow/red letter version. I did not find the input pad switch all that useful even with my hot output G&L L2500. I doubt I would have much use for it.
If you are looking for a cheap way to get a little more evening out, this would be a good option.