I used one on my previous bike for 18 months and loved it and I am now using one again on my new bike and am still loving it.
I got mine used but serviced and set up for me and the bike by J-Tech suspension, demonstrating that even if you get caught short with the (new) official UK distributor, there are others who can service and sort the unit.
You need to be aware that bushing hardware you need is different to that used in the Fox units (and others for that matter) so wherever you buy it from, make sure you get them to also supply the hardware.
In use, it feels very different to the RP23; where the Fox unit was wallowy and alwas felt under damped for me (my weight and riding, which is/was heavy although I have lost a lot of weight recently), the Vipr is very controlled. I never thought the RP23 felt plush; just mushy and vague. The Vipr is taught and doesn't give up travel easly. Consequently it can 'feel' harsh and less plush for those used to Fox set ups.
I have always thought it worked best when paired with a Bos Devile fork, which has a similar way of working and so when used with the Vip, balances the bike nicely. I have previously said that the Devile doesn't 'need' the Vipr but the Vipr probably does 'need' the Devile. I am not sure if I still agree with that, but I do think the fork is a little more versatile in what it's paired with.
I would wholeheartedly recommend the Vipr to anyone who is struggling to get an RP23 or other air can to work well for them. The only other damper that I have found gives as much control is the CCDB but don't think that the Vipr therefore works the same as the CCDB; it doesn't, it is very different again and if you like the CCDB 'feel', then it's not automatic that you will like the Vipr. But it will allow you to ride almost as quick.