Im currently driving the F25 X3 35i N55 with ZF8 and Xdrive - Its fantastic.
Less noisy than a diesel, plenty of power across the board, cool turbo woosh noises, and of course the fantastic 8 speed gearbox.
As reliable as any other BMW, the N55 just really needs good oil and regular oil changes, typical cooling system parts, probably the plastic charge pipe.
Nothing else major (Ive owned the N55 for over 2 years in the M135 remember?)
Fuel economy wise mine is sitting on 9.5L/100km for mixed highway/crawling traffic driving and I dont nanna it.