The original Infiniti M45 was designed for buyers who wanted something more than a G35, but something less than the flagship Q45. Though a good bargain, the old M45 just didn't have the style, features or interior room to appeal to luxury sedan buyers. It was a fast car, while its handling was firm but certainly not world-class.

After two years of sales, Infiniti pulled the plug and began work on an all-new M sedan. Well worth the wait, the new Infiniti car is a bona fide rear-drive performance sedan, still positioned between the flagship Q45 and the Infiniti G35 sport sedan. Looking much like a grown-up version of the G35, the Infiniti M45 sedan rides on an extended version of the G's rear-drive FM (front midship) platform. Unlike the previous Infiniti M45 sedan, the new M's interior offers dimensions and materials befitting a proper luxury ride. The standard interior is warm and inviting; the seats are firm and supportive; the standard amenities are plenty.

Its 340-hp, 4.5-liter, double-overhead-cam V8 is as good an engine as you'll find in any sedan. It provides plenty of low-end torque, freely revs to its 6,800-rpm redline and it's smooth enough to be in a car that costs twice as much. Power delivery, either out on the highway or off the line, is downright explosive. The responsive five-speed automatic transmission blips the throttle for you when you downshift and matches the engine revs flawlessly. The basic suspension setup is a double-wishbone front, multilink rear affair that, while tuned for compliance, isn't above taking on a stress-relieving two-lane blast into the foothills. The speed-sensitive steering is beautifully weighted, and never overboosted.

The Sport Package firms up the suspension without abandoning ride comfort, and adds Rear Active Steer which prompts the rear tires to emphasize turn-in when entering corners and enhance stability when exiting. Other Sport Package highlights include 19-inch cast-alloy wheels (standard are 18-inchers), climate-controlled front sport seats and aluminum interior trim. While the previous version never felt like a complete package, the new Infiniti M45 makes no excuses for its performance sedan intentions and feels like a hot rod next to the mild-mannered luxury sedans of the world. At the same time, a high level of style and luxury smooth the car's hard edges, and make the 2006 Infiniti M45 a must-drive in the midsize luxury sedan class.

The Infiniti M45 sedan is offered in two trim levels, base and Sport. Each one is furnished with leather upholstery, a 10-way adjustable driver seat with memory, one-touch up-and-down power windows, dual-zone automatic climate control, an auto-dimming rearview mirror, heated outside mirrors and wood trim. The Sport includes rear active steering, sport suspension, upgraded wheels, adaptive HID headlamps, aluminum interior trim and climate controlled sport seats. Options include a DVD-based navigation system, voice recognition technology and laser-based intelligent cruise control (when in cruising mode, this system senses vehicles ahead and adjusts your speed to maintain a selected following distance). Among the other extras are climate-controlled seats, adaptive HID headlamps, a lane departure warning system and a 14-speaker Bose Studio Surround sound system with digital 5.1 channel decoding.

The Infiniti M45 features a 4.5-liter V8 that pumps out 335 horsepower and 340 lb-ft of torque. A five-speed automatic with automanual capability and downshift rev matching is standard. At the track, that drivetrain gets the sedan from zero to 60 mph in 5.7 seconds, and through the quarter-mile in 14.4 seconds at 99.6 mph.

The M45 wears a full set of vented disc brakes, supplemented by ABS and the usual luxury car fare -- Electronic Brakeforce Distribution and BrakeAssist. Infiniti's VDC stability control system is also standard. All Infiniti M45 models also feature front-seat side-impact airbags, front and rear side curtain airbags and active head restraints for the front seats. An available lane departure warning system alerts the driver of unintended lane drifting.

The V8 engine in the 2006 Infiniti M45 provides gobs of low-end torque and is smooth enough to be in a car that costs twice as much. Power delivery, either out on the highway or off the line, is downright explosive. Braking performance is impressive, and a firmly tuned suspension provides excellent balance in the corners. The tradeoff is steering that can feel a bit raw around town and a suspension that crashes over large road irregularities, especially in the firm Sport model.

The M45's interior is well appointed and features ultrasoft leather seating and Brazilian Rosewood trim (aluminum on the Sport model). Its firm seats are infinitely adjustable, universally comfortable and heavily bolstered. Overall, the interior of this Infiniti car is well built and attractive, but its decor can be a bit busy.