After using flat urethane belting for the first time this year, we will probably never use round polycord again (except if we need a crossed belt somewhere).
Easy to weld (overlap, hit with a heat gun, and press with something flat to square the weld. Turn over and repeat). Doesn't require grooves (just build up a crown out of tape and the belt self centers, see:
http://www.lhup.edu/~dsimanek/scenario/crowning.htm).
Our rollers are just aluminum tubes with bearings pressed in each end.