Unfortunately it is is easy to confuse ultralights with microlights with experimentals with light sport. A U.S. ultralight can be built out of lawn accessories (Lawn chairs and mowers,) and be perfectly legal, they are the model airplanes of general aviation, in fact I don't know if they are considered general aviation at all. The big draw back with ultralights, as far as I can see, is that it costs a lot to make a safe-ish one, (the dacron sails aint cheap nor is AN hardware and ballistic chutes,) while the performance is so abysmal that i can't imagine them being that much fun after the novelty wears off. A lot of birds that look like ultralights, (sail wings, open tube frames, guy wires,) are actually light sport or experimentals, if I had the coin I'd love to have one of these beauties:
http://www.aircam.com/ which seems like the ultimate in fixed wing low slow safe flying.
There are ultralight versions of a few venerable experimentals but I don't really understand why it would be better to build them as ultralights than experimentals. I guess because you don't have to do ANY paperwork.