I recently drove a 645 convertible, engine sounded great, went fairly well, but overall a bit too much on the "soft" side for me.
Was also tempted by M6, 2nd hand coupes at £40k now, but think I want to stick with convertible, and they're a lot rarer and pricier - you also lose the lovely carbon roof for your extra money!
Am therefore probably going to change into a Merc SL55 AMG - the 500bhp supercharged V8 has seduced me!

They've also done a lot of depreciation by now, so hopefully that will slow down ...
Sadly, dealers are really shying away from Z4MR's at the moment, have had some ridiculous p/ex offers, or many dealers never even coming back with a price.
Have also not had a single response to an ad in Pistonheads, despite price having repeatedly been dropped, most recently to just £21k - for an '06 car in immaculate condition with 10,400 miles!