I have recently upgrade my hardware to a board that has 2 x M2 slots, what are your thoughts at using these for a cache pool ?, they seem to be quite cheap in comparison to ssd's, around £60 for a 500gb

M.2 is just a physical interface for SSDs. M.2 PCIE SSDs are generally substantially faster than conventional SATA SSDs. I currently use a bifurcation riser with two M.2 slots for my cache pool and performance is pretty great (though I do manage to bog down the less optimal 660p ssds I have occasionally)

The biggest concern is cooling. NVME drives run quite a bit warmer than their SATA counterparts. They don't need to be cold (in fact its bad for them) but they also need to not overheat.

I use NVMe drives as a cache pool, they're great.

 

You need to check your board specs to find out if it supports M.2 SATA or M.2 NVMe (PCIe). If it's just M.2 SATA, then it really is up to you on if you want to use them or not, they won't be any faster than your current SATA SSDs in terms of available bandwidth.

I'm running 2x 1TB NVMe SSDs in my two M.2 slots (btrfs RAID1) for my cache pool. I'll let my CrystalDiskMark results (over 10GBe) speak for themselves.