I recently switched to a jumbo and it's been fantastic. Bought a Ping Anser Milled putter and it comes stock with what has to be the smallest grip in history. After spending a ton of cash on it I was pretty hesitant to modify it. My putting went downhill very fast. Absolutely zero confidence. I could line up 10 balls on the putting green from 4 feet and miss 9. I added 6 strokes to my game.
Finally I took it in and though the size of the grip looked ridiculous, I went for it. First day out I got those 6 strokes back, and finally got that birdie on a par 5 that I always par or bogey on. You know those holes.....the one's that just have your number.
I can't see myself going back to even a midsize. Only drawback is a different feel for distance, but you get the hang of it after a few hours of practice.