Yes I have - well once anyway. There's a load of different oppertunities to get shots as the one I was at at least was fairly informal.
There's a tendancy - as I did to get as close as possible. I was crouched just to the side and behind one of the jumps. Great picture oppertunity for sure especially with something pretty wide looking up in the air with the horses over the top. I was in my element. However, horses are very large and are very unpredictable, as I was crouched by the side the pack jumped over, a bit of mayhem ensued resulting in the rider coming off the horse landing on its backside, again great action but its when the horse gets up - I assume on pure adrenelin that its no idea which way to run so it just bolted. It didn't get too close to me - but it could easily have done. So if you do want those stunning action shots of horses jumping over, just keep your wits about you and makesure you have an escape plan.
Other than that I had a great day, loads of action shots, was a cold day so loads of horses breathing etc etc. Used a Sigma 70-200, Nikon 18-70 and a 10-20 for those close scary action shops. Probably could have done with a little more reach. Also for me, horses, fences and grass all quite dark but the sky was overcast but still bright so watch your exposure, otherwise you'll blow your sky
HTH