I find interesting similarities between these pedals as I have tried each of them out in comparison lately and wanted to share my experience. Zendrive - the most open and uncompressed of the three without a loss of low-end. Just a great sounding pedal - quite possibly one of the best drives I have ever heard. The tone and voice knobs make this a really versatile pedal. Ultraphonix - Sounds very similar to the Zendrive, but with the low end rolled off - less versatile than the zendrive, but still a great sounding pedal. I do not like low-end rolloff so this gave way to the Zendrive on my board. BD-2w - a great sounding pedal and "somewhat" similar to the other two. Similar low-end roll-off of the Vertex, but overall a more compressed sound as you take the gain up. The Ultraphonix has better note separation I think than the BD-2w ("C" setting) though. I get what people love about the pedal, but the low-end roll-off and perceived compression are non-starters for me personally. All three have their place, depending upon what tone you are looking for. I ended up with the Zendrive and overall, was surprised how they were all more similar than not in their sounds. They are different enough, but I personally would not have two of these on the same board.