my 2p having just sold my Mk 1 Elise (111S model) which I owned as my daily driver for 4yrs, the longest I have ever owned a car!
Have to echo snowfish's comments regarding driver feedback on the elise. I know the 'M' has hydraulic assited steering but driver feedback relative to the elise is poor. The elise inspired confidence because you knew through the wheel and brake pedal (and yer bum!) where the limits of adhesion were. That combined with the fact a mk1 elise is half the weight of a z4 meant the elise was a far superior (and fun!) driving experience on a 'B' road compared to the Z.
On a recent such B road blast in the M which i frequented in the Elise, i arrived at my destination unruffled and notibly less excited and satisfied than i would of been if I had driven the elise.
Other plus points for the elise were 35+mpg, cheap to run (unless something breaks - which will happen eventually), and generally well liked by public and other motorists.
That said the practicalities of elise ownership dont compare to that of the Z4... entry/exit, boot space and all the other things drivers of 'normal cars' take for granted. Where the Z4 excels is straight-line pace, driving comfort and noise (ive got a gruppem and supersprint!

. Owning a Z also now opens up the posibilities of pan-europe hoons that I have shied away from in the past due to the Elise being uncomfortable on long motorway drives.
Im not knocking the Z4M. Its horses for courses... if you want sheer raw driving exhilleration and fun, then an elise is your ticket.
I switched to the Z4MC as, alas after 4yrs of elise ownership i was looking for something a bit more practical, but still hauled ass, was devastatingly pretty to look at, and exclusive enough that I could go for a week without seeing another one on the road.