Oooo. Those look nice.
I recently bought a set of Maxxis Velocitas that were probably mispriced online at $6. Never the less, they showed up in my order. They look paperthin with 60's dragster-looking yellow "MAXXIS" written hugely across them. Someday I'll stumble across an equally cheap set of tubular rims or wheels and use them.
My only experience with sewups was a pair of green Rallys I installed on the first fixed gear wheelset I built in '00. They were installed on a Raleigh Sports, complete with fenders and chainguard. Needless to say, that experiment didn't last long as I learned about the importance of lockring tightening