Pat Parelli is wonderful, but so is John Lyons, and Clinton Anderson, and a ton of other trainers I love to death.
I think it all depends on the person's natural style, as well as their learning style. Its the same as when we are trying to learn something in college, everyone goes about a different way, and oftentimes, trying someone elses way just doesn't work for us.
For me, I like Pat Parelli/John Lyons/Clinton on the ground, and then Clinton Anderson/Bob Avila/John Lyons in the saddle (yup, I use all their stuff.) I mix and match babe! Lol.
In any case, I always start a horse with Parelli's games. It makes ground work fun, and it really does get the horse's moving in all directions with just a suggestion from the ground. By the time I get in the saddle they know pressure in certain places means to move in the same way we did on the ground. They respect and trust me by the time saddle work comes, and they also are excellent loading into the trailer. Even with trained horses, I play the games a bit to see what kind of mood they are in before I get on. Parelli calls them pre-flight checks, and they help me work out the horse's mental issues of the day before I climb aboard.
As for in the saddle, I've never really tried alot of Parelli's stuff. I dont know... it just never appealled to me so much. I worship his ground work but I'm still true to John Lyons and Bob Avilla. I think mostly I take lessons/advice from many well known trainers and their videos/articles and use them if I like them and if the work for me. End of story.