I had a couple of pneumags. It does make the trigger pull nice and light, but not electro-light and only just about walkable. It's also expensive to do - especially if you want to have all the pneumatics concealed, as you'll either need a frame designed for pneumatics or have an intelliframe modified to take the bits and also modify your ASA. You'll also need a suitable LPR and all the other gubbins - most of which is usually only easily available from the US.
I've had both a 'normal' pneumag and one using the cheater manifold. The cheater combined a couple of the bits into a single unit and was easier to install, but I actually got better results using the standard MSV2/MPA3 setup.
I'd agree with renie that given the effort and cost, it's probably not worth it - if you're looking for high ROF you'd get better luck sticking a Ninja SHP reg on your bottle to ramp up the reactivity of your X-Valve. For the amount I spent on building my shiny Pneumag below, I could probably have got myself a new Ego.
Here's a pic of one of mine - had a pneu-ready M90 frame, a CP direct mount ASA which I modded myself to feed air direct to the LPR, Palmers micro rock LPR, MSV-2 switch and MPA3 ram.
And here's a vid of the cheater setup I had - you can see it's much cleaner inside the frame, although that's mainly because the LPR is hanging off the side of the ASA!
