Hey! I have a vamoots, psychlo x ybb and 26" ybb. Love the px ybb. It's an awesome bike. I'm in WI and a lot of our cross races are on grass and it really saves you a lot of energy during the race. You can ride behind guys who's arms and legs are jiggling all over the place and you're just cruising. I've never locked either ybb out. I will say that you can feel the suspension when you're sprinting at high speed, but, I also think it helps the rear wheel bite.
As I've been reading these posts, I think I'm going to get a set of those challenge 27's like Jon's cross bike and I'm going to dedicate this one to the trailer, which gets a lot of miles and this will make them very enjoyable.
For what it's worth, I also enjoy top mount cross levers, and miss them when I go back to my road bike. They're just comfortable, but seem to have gone out of style. Love barends on the mtb too.. ;-)
I always watch the internet for a vamoots ybb. That would be a super fun bike to add to the stable!