Joining up worked wonders for my boy!
When I got him as a five year old, he had spent the last 4 years of his life in the pasture with little human contact. He was hard to catch, and extremely untrusting, with no confidence in himself or humans.
When I got a trainer for him, she did joining up for weeks before she started any ground work on him. I was disappointed at first, to me it looked like she was just having him run around a round pen. I didn't understand the complexity until she had me start doing it myself. Sawyer, within a few weeks of joining up before his lessons, he was changing fast. He began following me on the lead line with no contact, coming up to me in the stall, and just trusting me.
We still do joining up in a large arena now. I can send him around any size arena with no whip. I can (most of the time) control his speed, stop him, and have him come into me. He'll follow me around the arena. Backing up when I do. He pivots on his front end is a walk at his hind quarters.
In all, I believe this is the most important thing he learned in training. I remember one day, I was taking a hike with Sawyer. He couldn't be rode at the time because he'd gotten a nasty kick in the leg from another horse. He could walk on his lead though, so I'd take him out to grassy trails to let him eat and relax outside of his stall. His halter broke on the trail, and I was very nervous about getting him home. I originally put the lead around his neck, but noticed he was just following me. I ended up walking him almost a mile home with no lead what-so-ever, because he was so willing to follow me. I took him straight to our arena and did our joining up and let him play.
I find it very valuable and always replace lunging with joining up. It's a mentally and physically engaging exercise, and really improves your relationship with your horse.
"Putting pads on a gaited horse is like putting a mask on a beautiful woman."