This is my "New Adjustable Carbon Fiber Seat and Headrest" review - for the S40.
While I would give this seat two thumbs up for the V20, I have to say the S40 version is not ready for prime time. I bought this seat used from Al_D who discussed the problems in several posts within this thread. The main problem is the lack of butt support. At a 20 degree angle on the V20, your weight is more evenly distributed across the seat. At a 40 degree angle on the S40, your weight slides down the seat, and there just isn't enough seat surface down their to support your butt. The result is an uncomfortable ride where you feel like you're sliding off the seat. At least, that was my experience after installing it.
Al_D has noted that the butt extender plate that is included with the stock S40 seat would be the perfect solution if only it would fit right. Although the mounting holes are in the right place, this bottom plate is formed to fit onto the stock S40 seat, and when you install it on the new adjustable seat, there is a 3/4" gap between the edge of the plate and where it meets the seat, which extends horizontally across the seat. This ridge would not work, but by tightening down the mounting screws, you can bend the extender butt plate down to meet the seat (if you dare). Will this cause the carbon fiber extender plate to crack? Mine hasn't so far. I tightened it all the way down. There is still a 1/4" gap across the seat, but the Ventisit pad makes it so I can barely feel that ridge. It doesn't really bother me. What bothers me more is that a $700 seat is being sold specifically for the S40, when it doesn't really work for the S40 without a but plate, which is not included. There may be some people who are riding their S40s with the stock seat, without the butt plate. If you are one of these people, the new adjustable seat will be perfect for you. But I definitely need the plate, and while the stock butt plate doesn't fit right, it's working for me.
Initially I adjusted the seat to the same over-extended length as my new V20 seat. For some reason, it felt to high in my shoulders, so I ended up adjusting it 1 mounting hole shorter than the V20 setting. I mentioned in my review of the V20 version that I really like this seat's shoulder support. I wondered how it would be in the more upright S40. I bought my S40 first, and later bought my V20. When riding my S40, I never used the headrest or felt I needed one. The stock headrest was there, but I never adjusted it so I could rest my head on it. When I climbed into my V20 I realized the the more reclined angle made a headrest a necessity. So when I started riding the S40 with the new adjustable seat, I realized that now that my shoulders were actually supported by the seat, the headrest made a lot of sense. So with this new seat, the top of my torso leans back further, and the seat feels more reclined than my original S40 seat. Maybe it's because I was sitting in it wrong, but my torso is so long, that the stock S40 seat (and V20 seat) miss my shoulders altogether. They never touch the seat. Now on both bikes, my shoulders are supported, and it feels good. On the S40, it is changing my riding posture (for the better).
The large Ventisit pad works well with the adjustable seat on my V20, but it is about 2" short on the S40 due to the extended length of the butt plate. So, in my opinion, not only should Cruzbike design and include with the S40 version of this seat, a butt plate that fits the seat, and an option for an XL size Ventisit pad that is 2-3 inches longer than the Large pad. I am making the large pad work, but it would be nice to have another inch at the bottom and top of the seat.
The headrest that comes with the new adjustable seat is the best one I've ever used, and so it was easy to get it perfect. After getting everything dialed in I rode another 25 miles, and I'm really happy with the fit and the comfort. Another game-changer. I have 2 S40s so I have to decide whether to spend another $700 for another adjustable seat. If anyone is unhappy with your new adjustable carbon fiber seat and wants to sell it, let me know.
My bottom line is, this is a great seat for the S40. I don't think it is ready for prime time until the butt plate and long Ventisit pad options can be included, but I have made it work for me, and I will be getting one for other S40 as well.