Really the only problem with the e90 m3s so far is the rod bearings.
They all use about a quart of oil every 4000 miles or so.
The maintenance will be much,much, higher than your NC Miata.
Gotta use the 10w-60 weight from BMW, and it uses 10qts of it.
I get around 18mpg in mine with combined driving, but its the best car I have ever owned.
Rod bearings seem to be BMW's weak point. S62, S65, and S85 all suffer from rod bearing issues.
I really gotta get an E92 M3 6 spd and try it out.