Ham is building up another incredible sled, the Ibis Mojo. I will admit, I had my reservations about this bike when it was introduced, but even just a parking lot test will tell you that this is a great ride. Granted, this one has a Push'd Pike and RP23, so it has the best air suspension components available.
What a ride! Fred Cuthbert makes a killer ride. By far the best 29er I have ever been on. The geometry is a close resemblance of what you see out of Jeff Jone's stable. It eems to put you in the perfect stand and hammer climbing position that you are often forced into with rigid singlespeeding. The bike almost felt like it had suspension with the large volume tires and compliant rear end. Nothing quite beats the feeling of a custom steel ride.
Everyone that is saying that 15mm is going to be lighter than 20mm has no engineering background at all. All things created equal...the difference with the same materials and parts would at most be several grams...and this small difference would not be felt on the bike when it is actually ridden.