I've only had one weights session when I was feeling a little worse for wear from the night before, and it was really bad. Cardio, on the other hand, is the best hangover cure in existence (especially as it makes you hungry too and wolfing down food makes you feel much better). When I've erged/cycled after a heavy night I feel amazing afterwards, even if there is a point a few minutes in when you feel really dodgy. Push through it and the overall effect of the exercise is to make the hangover better rather than worse.