I had a B & M one...
Basically, it does shorten the shift by quite a bit and has a nice notchy, clicky feel to it...
...however, that's if you can actually get it to fit so that you can engage ALL of the gears! Mine was a bloody nightmare and never was right in all the time I had it, you'd adjust it one way and get 1-6 but not reverse, or 2-6 + R but not 1st, etc etc. In the end I settled for being able to engage all gears, but having to crunch into (and sometimes miss) 6th and reverse!
Also, although throw was reduced by the kit, it made it very stiff and often a pain for day-to-day driving. I liked the shortness of the throw, but it wasn't worth it IMO.
Oh and did I mention the build quality of it? Well when I say that the shifter snapped off in my hand on a trackday, does that explain it? Basically, the reason it broke off (we discovered) was because the shaft was connected to the base with the smallest blob of weld, off-center as well - very poor.
I only had my stock shifter re-fitted a couple of weeks back, I'd been driving around for nearly two years with it held on by a £5 weld that some garage near Goodwood bodged on for me - thing is though, it was obviously stronger than the original!
Other than those points though, it was alright