Anyone have them, or have any experience of them?

Trying to weigh up between a new set of these or a used set of mono m4’s

No idea on Mono M4 but I’ve just swapped a Juicy 7 for an Elixir CR, it’s notably better than the J7 – a bit more powerful, but way easier on a practical level – tool-less reach adjuster, alignment is easy as you can see the pads & rotor from the top, and you can also change pads from the top (not tested this bit yet). I was also surprised to find the carbon lever grippier under the finger than the alloy one on the J7, though I don’t think I’ve ever slipped off the alloy lever. No idea on longevity though.

its a choice between used mono m4’s at around £100

or new old stock elixir cr carbons at £140

Where are these old stock elixirs?

I changed from Mono M4s to Elixir CRs on my DH bike. Admittedly the M4s were the old lever design (The “Tech” levers are a lot better) but the Elixirs are in a different league as far as power is concerned. They’re a little more fiddly to set up & bleed but are much more powerful and just as progressive.