I have the GP20 cartridges fitted to both my 1965 250cc Ducati racer and my '97 900 and all I can say is that on both bikes they have been amazing.
It sounds a bit odd but I got the race bike done first to try to give it a more recognised feel when I started classic racing and so got Maxton to make some rear shocks for it too as the Hagons it had fitted were way past their best. Now it handles like a dream- a fifty year old dream (drum brakes an' all) but completely different to how it did when I got it.
On my Monster I have run a Nitron rear shock for about ten years (have owned the bike seventeen) and over the years have played with various set ups and oil weights some with more success than others nut none perfect. So after the race bike revelation it was an easy choice to get Maxtom to work their magic once again and I can honestly say I wish I had done it years ago- forget about how they might be on the track, on the road the bike feel fantastic. No more kicking off of bumps or feeling like the front is going solid on the brakes. In addition to that they actually give a much more comfortable ride making long distances a real pleasure.
You even get a T-shirt:
Only the fork caps are a give away to anything special inside:
