We have a couple of the cheap plastic sledges which are pretty fast, but not at all steerable and break easily. I wanted a sledge which was strong, steerable and fast. I chose the waste pipe to give a minimal contact area on hard snow but a larger area as the snow deepens. The tube is also slightly flexible to allow bending for the steering.
UPDATE - This sledge works best in hard-packed snow. To make it work in deeper snow, follow BugsyandSpike's suggestion below and put a 2" x 1" piece of wood (with accordingly longer bolts) between the platform and the tube to give it better ground clearance. (There's a few other worthwhile mods in that comment too.)
It has been mentioned that PVC gets brittle and can splinter in the cold. I've given my sledge some pretty rough treatment in a couple of degrees below with no ill effects, but it's one to be aware of.