IMO, the ZW overdrive is very poor for the money. I was looking for an overdrive to both boost, and to work well alone also.
I bought the ZW, played it for two days, and then returned it. It was noisy, and it made a very annoying popping noise whenever turned on and off (not at all usable to smooth gigging). I actually didn't like the overdrive very much either.
I tried the stock Boss SD-1, and it was like every other boss pedal.....good, but missing a few things.
Then, I stumbled upon Keeley, and I've never turned back. The keeley SD-1 is an amazing overdrive, and really pushes my rectifier into crazy overdrive at low volumes. It's an amazing boost, and works perfectly on its own too, for a very smooth and creamy overdrive.
IMO, Keeley is the way to go!