You probably haven't heard that from anyone who actually shoots one regularly. They are reliable, lightweight, use AR mags, ok accuracy normally in the 2-3" range (although my "A" model is a 1.25" gun with one specific load), easier to clean (don't get as dirty), folds for compact storage, excellent warrenty, although I've never had to use it. As far as cheap, yep, they use plastic, but it is steel everywhere it needs to be. My "A" model has been folded and unfolded a lot with no signs of any wear in the mechanism so far.
I've got an "A" model that I've had for a couple of years, I don't count rounds but it's had a lot of 30 round mags put through it. Cheesy front sight but I put optics on mine anyway. I have not had a single FTF or FTE, no problems of any kind in 2+ years.
I like it so much I bought a "C" model at the December gun show in Houston. I like the sights better, the slightly shorter and stouter barrel, and it's threaded. Only about 150-200 rounds through this one but once again, not a single problem and it looks like it will be a little more accurate than the "A" model on average.
FWIW I also have several AR's in .223, 6x45 and .50 Beowulf. I like them also, but the SU is just plain fun to shoot. AR's have some advantages over the SU, lot's of stuff available to hang on them, interchangeable uppers, can be more accurate, probably stouter overall and probably able to withstand more punishment. My .223 bull barrel gun witht he JP trigger is much more accurate than the SU, but I've got considerably more money in it than an SU.