Not so much a bad experience. I had a course of antibiotics at the end of April '07 and my first few races after were very very hard work. I think that the antibiotics really gave my body a kicking, as I only had an ear infection. But they were pretty strong ones according to my pharmacy student friends. I think they push you back in order to allow you to get stronger again if you see what I mean.
I guess its all down to personal tolerances. But I find I have trouble with most medication when it comes to cycling performance. Like wise after a few antihistimines after getting bitten at Palace (which swelled my ankle up pretty bad), I seemed to be in a rut for a good few days. It was the first time I had taken them, so I will think very carefully about taking them again. So I guess the trick is to just go by how you feel! It's a classic line that one huh...