From brief experience it can work well because you opponent wont have a tense stomach and will often walk onto it. Just keep them thinking.
I've seen Mayweather use it very effectively because of his long reach. The way I was taught to throw it you must get down low so that your shoulder is in line with the target (bendting at the knees and also at the waist), and also slide your right hand (orthodox) from you chin to the front of your face (just in case you have to catch a punch).
He lands with it. It's used for multiple purposes. First and foremost to set up other punches. He can steal jabs to the body and then feint those later on to set up punches up top. He also uses it to stop guys from coming in, or at least give them something to run into. A body jab (especially if used to the chest region) can be a stopper. Not a fight ender but a body stopper. Used correctly it can prevent your opponent from advancing and is just another way for Mayweather to keep range.
He can set up head shots with it if he doesn't step in. If he steps in he is usually too low (to avoid incoming) thus out of position for head shots.
I like looking downstairs, but punching upstairs or vice versa. Sparred a southpaw yesterday, I bent and looked downstairs as if I was going to throw the right hand to his plexus, but came up and smashed him in the nose with it
I jab to the body all the time. Slip opponent jab and throw a jab to the body. Jab to the head then duck and jab to the body. Feint a jab to the head then duck and jab to the body. etc It's not a major part of my offense and I don't use it as much as Floyd, but I throw all my punches to the body (and head of course)
I use it all the time. 2 quote roy jones during the 1st pac morales fight a gud pro knows how to go up and down his opponents body. The jab to the body keeps him guessing and creates openings to the head.