I don't have any actual weights, but my 18" d.i.d. "U" rim certainly felt lighter than my 18" d.i.d. "Dirtstar" when I was changing tires a couple weekends ago. The "U" rim has no visible flat spots either which is more than I can say for ANY other rim I've used.
edit: I guess I should have clarified that my DirtStar is a 2.5" and mounted to a cush hub, so of course it was heavier and not an apples to apples comparison!