The Quick-Shifters - Seamless Up Shifts
First, not all quick-shifters work in the same way, but they do all try to allow the same thing: full throttle seamless up shifts.
There are basically two types of quick-shifting systems. Some units are powered, in other words, you only need to tap the gear lever to change up and the quick-shifter system does the rest, others just kill the ignition of fuel to unload the transmission and allow your foot to slot the next gear. The powered units are often air driven and unsurprisingly, they're a lot bulkier and heavier than the electronic only quick-shifter units.
As for the one to get, Dynojet latest offering is brilliant. If you already have a Dynojet Power Commander III on your motorcycle, it can be plugged into that and set-up relatively easily. It's definitely the way to go.
If you don't have a Power Commander III, then you can get a complete unit with its own power supply from a company like Translogic, which is well worth a look. They've been used by many top Superbike racing teams.
Both the Dynojet and Translogic quick-shifters are electronic only, but can still save you up to a meter on every shift. Although far smoother and more civilized than they were five years ago, they're really only worth splashing out for on a race track racing motorcycle, although they are hugely entertaining on the public road.