For what it's worth I had a slipper on my GSXR600. Unless you're on a racetrack it's unlikely you'll ever need it. You've got to downshift VERY aggressively - like 3 or more gears at a time for it to engage. In 7500 miles on the GSXR I never once needed it. But your riding habits may be different. If you're constantly skidding the rear wheel because you downshift three times and just drop the clutch - you're in the market for a slipper.
I take pride in my ability to match revs and downshift smoothly. Although, it would be a cool gadget to say I have....maybe if my next bike has one it would be cool, but I don't think it will be a selling point.
I ride a lot of country roads with roundabouts in them and I thought it would be fun to be able to carry speed into them and do my shifting before I'm in the middle of the turn.