I have a JHP Ducati one fitted to my old Senna by the infamous Nelly.
When I first tested it I had to look carefully to check she had changed gear it is so smooth, a staedy increase in mph and slight drop in revs were the only real clues.
One of the best mods I ever made for trackwork, particularly Nurburg where some corners are so long in places you need to change up mid corner and trying to get a handfull of clutch or simply rolling off to chage up can be quite interesting, good if you like wrestling crocodiles, bad if you want a smooth exit on full power.
It is of little use on the road unless you just want to blast down a straight road and change gear easily, and remember to switch it off when not needed. It can be a little embarrasing 'po-going' down the road as your left foot hits and misses the gear selector just enough to break ignition but not enough to change gear.
I would highly reccomend one if you do much trackwork, but would say no for road use, my '2 pennoth' worth.
Cheers