The M45 uses the full-size Q45 chassis, but its overall length is three inches less. Legroom is still decent in front, but five inches have been lost in the rear. Wide rear doors make entering relatively easy, however.

The exterior dimensions of the car are a bit unusual; it's relatively long, tall and narrow; a 2003 Honda Accord, for example, is a couple inches lower and wider. So the M45 has good headroom front and rear (although our rear-seat passenger said the roofline made him feel claustrophobic), but it's squeezed on shoulder room. We first encountered this when we reached down to adjust the standard 10-way power seat; our left wrist nearly got wedged between the seat and the door.

The front seats are on the firm side: not uncomfortable, but not especially relaxing, though power lumbar support is always nice to have. We might call them sporty if there were more bolstering.

Heat in the front seats is standard. An optional cooling system for the seats blows cool air through the pores in the leather. It sometimes feels like you have a hole in your pants. It should be the hot ticket in humid climates, especially during that period before the interior cools down.

The M45 offers voice recognition, but we had trouble getting it to understand us. We also did not care for the leaf on the LCD screen of the Vehicle Information Center that indicates instantaneous fuel mileage; when you're coasting it's green, and when you get on the throttle the leaf progressively turns a smoggy gray, evidently to remind you that your 340-horsepower automobile is polluting the environment. The actual mileage numbers are there, but they're so small you can't read them.

The driver's space is tidy and sporty. There's a great four-spoke steering wheel with a nice thick rim, fairly small diameter, and controls for audio and cruise control. Four gauges, a big speedo and tach, smaller gas and temp, are laid out as cleanly as they come, and are backlit at all times. On the floor there's a solid dead pedal, and on the door there's an armrest that fits perfectly. The dashboard slopes pleasantly toward the windshield, with reasonable dials and buttons including a control for the information center, and the Infiniti trademark clock is small and discreet. There's a modest amount of smoky maple trim, most of it on the console.

In addition to the two-stage airbags, important standard safety features include side thorax airbags in front, curtain airbags front and rear, and active front headrests.