The key difference between compact sport sedans, like Infiniti's G37, and mid-sizers like the 2012 Infiniti M, is back-seat space. There's plenty of space for two adults in back in the M37 or M56--or a fifth on occasion--and the seats actually feel sculpted to the human form.
In front, it's exceptionally comfortable; both driver and front passenger get ten-way power seats, and the front cabin has good head and shoulder room, minus a little width for the thicker center console.In back, the only caveat is that the tall doors and rear deck can give the rear occupants a closed-in feel. The tallest passengers might find it a little light on headroom, but the 2011 M doesn’t sacrifice rear-seat comfort for its looks, as do the Jaguar XF.
A big trunk rounds out this package that's surprisingly practical for a sport sedan; it's useful though not all that long; otherwise there's good cabin storage, with a shallow-ish console augmented by a deep glovebox.
Ride quality is very good, although as with the somewhat smaller G37 there's a bit more road noise inside the cabin than you might expect for a sedan that emphasizes not just performance, but luxury.
The V-6 version has the noticeable, vibration-ridden feel of a modern-day Nissan, and it needs to improve. At wide-open throttle the M37 is loud and transmits a lot of unwanted shake into the steering column and floorpan that’s sure to be noticed by any buyer in the $50,000 sedan segment.