They just wernt designed for the UK market it was made to appeal to the US market. Although they are great they just arent the same and super thirst when golfs and minis will be barking at your heels. In a straight line they are balistic and great fun but like I said you really have to be at 4k+ for it to feel fast whereas the S54 has 90% of its torque from 2k.
If they had put the V8 in another shape I think it would have made for some great cars just the E92 is a bit of a miss for me. I grew up with E46 M3s and E92s and the E46s were by far my favorite. Much more fun to drive.
The Z4m roady is literally the M car bargain and IMO i cant really see them depreciate much more. For example the only thing comparable for the money is an E46 M3 vert and they are absolutely awful, truly awful to drive the whole chassis flexes over every bump. Or a super leggy E46 M3 SMG, there are no manuals even close to 12k with 65k on them.
The Z4M being designed from the ground up to be a roady its better in every respect. Similar to the Z3M it will take a lot longer to start on the up curve so I cant see them appreciating any time soon but hovering around where they are. So you could leave and come back without much penalty. There will be a time soon when the penny will drop and they will start appreciating.
The thing with every car is its worth what someone is willing to pay for it. I understand the VX is ballistic but it cant touch the Z for a well rounded package with the prestige, comfort and build quality compared to the kit car feel of the VX.