I did one in June and enjoyed it, but I don't feel that it's a true measure of a good trail horse. 

Read up about all the obstacles and have an idea of how to do each one.  It's better to try it and then if your horse won't do it, stop.  Those that kept trying and trying, got marked down and you get a zero if time runs out.  The ones my horse did partially, I at least scored some points.

  I rode with 2 friends and it was ironic to us that the youngest horse who isn't that great of a trail horse, scored higher than our 2 seasoned trail horses.  My horse isn't a great backer and that's where I lost points, but he is wonderful out on the trail itself, led our group the entire time, is quiet in traffic, safe, sane, etc

   The young mare that is not really all that great out on the trail, does back and though she acted up really bad at the obstacles (reared), scored 2 points higher overall...  go figure...  I'll take my guy anyday over her. I'd rather be safe.