I had been debating purchasing a JumboMax grip for my driver to try out for a couple months now. I had a Golf Pride Jumbo on my previous driver and liked the extra thickness it provided. I am a tall guy (6'5") with average size hands for my height, so normal sized grips, and even midsize, just feel very skinny to me.
Two weeks ago I purchased and installed the JumboMax JMX UltraLite in a large, which is a plus 11/32" over standard. I didn't want to go larger than this as I wanted something thick, but not monstrous. I have since played two rounds with the grip installed and I am driving the ball straighter than I have since I upgraded my driver back in May. I am consistently hitting baby fades which is my go-to shot. My main miss with the driver is a slice, and in two rounds I have no hit a slice (couple bigger fades but nothing even close to calling a slice). It has only been two rounds, so time will tell if it is actually the grip or if I am just in a zone with my driver right now. I do really like the feel of the grip. It feels much more natural for my larger hands, and I feel as though it increases my ability to keep the club face consistent through my swing.
My two hybrids are in need of a re-gripping, so I will try the JumboMax out on those clubs next. It will be a while before my irons need to be re-gripped.
Just wanted to post a short review in case anyone else was interested. The most recent posting on this site related to JumboMax was a 2017 comment on a 2011/2012 thread. Side note, I was interested to see that these grips have been around for much longer than I thought. I think Bryson has been playing them for quite a while now, but I know I just recently heard of them within the last few months.