A friend of mine used to shoot IPSC with his USP in .40. He did just fine with it, though magazines were tough for him to find. Those of us shooting Glocks were always loaded to the gills with high-caps, but he only had a couple magazines for the USP--which meant he was always furiously re-loading in between stages.
I've always liked the Glocks because they are simple and reliable. Maintenance is easy. Other than that, I guess it's just personal preference.