I've got long fingers, and have often felt like I just wasn't getting the best grip on the club, even with normal jumbo grips. Recently I decided to buy and test out a JumboMax grip (size L) on a 7-iron that's similar to the one in my set. The results are promising! Here are a few thoughts:
(1) More of my hands are actually in contact with the grip now, which is helping me feel the position of the club face as I swing back and through.
(2) I notice less tension in my hands and forearms, which allows me to take the club back and swing it through without thinking so much about grip pressure, how I'm holding onto the club, or how I need to manipulate the club face for a square strike.
(3) Thicker grips can make it tougher to square the club face. During my first practice session, I definitely left the face open a touch on a few shots. (Normally, my miss is to turn the club face over and hook it a little.) So there will be a bit of a learning curve, at least for me.
(4) I noticed a slight reduction in feedback compared with my other grips, which might simply be a result of the greater thickness of the JumboMax grips. It's not a deal breaker by any means. I just like receiving feedback when I contact the ball. If you look at it another way, it could be viewed as a plus, because the thicker grip can dampen more vibration on off-center strikes.
Overall, I'm very happy so far, and I think after more practice I'm going to want to regrip my entire set with JumboMax grips.