Is it exciting? yes.
Is it dangerous? (for horse and rider) Yes
Is it cruel to horses? Well, think about it. What are these horses worth? More than you or I will ever see in a lifetime, in some cases. So I do not think owners, trainers or jockeys take it lightly. Horses run becasue of the herd instinct. Way back in the primitive horse that was based on fear (So is One Man and his Dog based on the fear of the sheepdogs by the sheep - do we regard that as cruel to sheep or do we say "aah, traditions of old England)) However, horses which are bred to run either on the flat or over hurdles have it in their blood. The Jockey Club keeps a very close watch on jockeys, to discourage and punish any over use of the whip. (It is actually the movement of the whip which the horse glimpses out of the corner of his eye which drives him on) Any jockey actually beating his mount would find himself without rides very quickly and out on his ears.
So I will be watching - even if from behind the sofa when the excitement gets to me - and celebrating the beauty and speed of the horses and the skill and courage of the jockeys.