They are a nice pistol but I think the Glock 19 is their main competitor. I can't get too excited about the 9mm USP Compact. The $ difference between the two pistols is pretty big. Why spend big bucks for a mere 9mm pistol?
I think the .40 caliber and .45 caliber USP Compacts are probably more popular because they are unique for a plastic framed pistol - thumb safety, hammer, decocker, variations, etc.
That said I own a .45 USP Compact and 9mm Glocks.
Hope this rambling helps.