Horner was winning everything in the States back in his Webcor days. The guy was an amazing rider. When he went to Europe, well, things didn't go so well, though he did have one victory at Saunier in the Tour de Suisse I think. I seem to remember he almost won Romandie one year, but he lost it in the TT on the last stage.
As I said, my impression is that he was racing clean in the States, then decided that wasn't enough $$, so took his talents to Europe late in his career where he went on the juice. I remember he raised questions about US Postal long ago.