The adage goes that "bigger is better", and it seems that automakers have taken it to heart. For 2006, BMW responds with its 5 Series: the 545i of 2005 sheds its 4.4-litre V8 in favour of a new 4.8-litre, and becomes the 550i.

It's a considerable step up, with horsepower increasing from 325 to 360, and torque ramping up from 330 lb-ft to 360. What doesn't change is a long list of features, and some very serious long-haul comfort. I had the opportunity to take the 550i on a trip of four and a half hours, and got out of its leather seat feeling like I'd been behind the wheel for forty minutes.

The 550i is one of three "regular" models in the 5 Series, along with the six-cylinder 525 and 530 models; the 5 designation tops out with the mighty M5. (The odd numbering system no longer reflects engine sizes, as both the 525 and 530 use a 3.0-litre; now, the scale just moves up in increments of five as the engines increase in power.) The 525 and 530 are available with xDrive all-wheel-drive, but the 550i is strictly rear-wheel. The base six-cylinder starts at $58,600, but moving up to the eight-cylinder means a starting price of $78,600. My tester was further bulked up with options that sent it to a cool $91,100, before taxes and freight.

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Situated between the nimble 3 Series and the bank-vault-quiet 7 Series, the 550i marries the best of both of them. The new engine is also shared with the 2006 650 and 750, but in the case of the 6 Series, that brute strength is dropped into the 550's more practical four-door package that offers a real back seat in place of the 650's 2+2 configuration.

A six-speed manual is standard equipment - BMW is one of the few luxury manufacturers that still puts stick shifts into its bigger vehicles, which accounts for much of its popularity with driving enthusiasts - but my tester was equipped with an optional six-speed automatic. It was a mixed blessing: the 5 Series is made for taller drivers, and at 5-foot-4, I often have difficulty maintaining a comfortable driving position when I have to reach the clutch in the larger BMWs. But the automatic shifts slowly under moderate throttle; it works much better when set into the Sport mode, which makes for much crisper shifts, and best of all when shifted "manually" via the Steptronic mode.

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To my surprise, my tester's throaty V8 stumbled a couple of times when it was very cold. Once it warmed up, though, it was a pleasure: it has monstrous acceleration, and it eats up real estate in a hurry (official figures are zero to 100 km/h in 5.7 seconds with the automatic - 5.6 seconds with the stick - on its way to an electronically-controlled top speed of 240 km/h.) Fuel mileage was also more efficient than I would have expected for the performance, returning 11.0 L/100 km on the highway, and 14.5 L/100 km in the city, by my calculations.

Like all BMWs, the 550i feels heavy and solid, but never cumbersome; while most of its Japanese competitors feel lighter on their feet, the 550i maintains a heft commonly associated with Teutonic sedans. The turning radius is tight, cornering is flat, and the car's tendency to push its rear wheels out slightly on hard turns is instantly brought back in line with the Dynamic Stability Control that's standard on all 5 Series models. The ride is big-luxo-car smooth, even with my tester's optional 18-inch run-flat tires.

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My tester was equipped with Active Steering, which has as many detractors as it has adherents. A planetary gear on the steering column increases the steering angle at lower speeds, and reduces it at higher speeds. It requires much less effort when parking or manoeuvring, but the trade-off is a somewhat artificial feel; you'll need to test-drive with and without it, to see if it's worth the extra $1,500 to you.

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Inside, the 550i is a well-fitted and elegant combination of soft leather, padded textured plastic and steel-grey "Anthracite Maple" inserts. My "Comfort Seats" were part of a $4,500 Premium Package, allowing me to adjust my seating in multiple configurations, including the backrest angle and seat depth. The package also puts two-stage heating into the two rear outboard seats.

On the plus side, there's a heated wheel with power tilt and telescope; a "residual heat" setting that blows hot air when the engine is turned off, so you can sit in comfort while waiting for someone; dual-zone climate control; elegant theatre lighting; and on my tester, an optional Head-Up Display.

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It's pricey at $1,500 (and even more expensive should you break your windshield, as it requires special non-reflective glass), and it does create an ugly cavern in an otherwise smooth dash, but once you've used it, you'll want it. I thought it would be a distraction, but the hovering speedo numbers integrate seamlessly into the line of vision; should the car be equipped with a navigation system, upcoming turns appear on the windshield display as well.

The minus side? Chalk up an awkward inside door handle placement, which requires that you push the heavy door open with your elbow once it's unlatched; too-small map pockets; a silly engine start/stop button that must be pressed once the key is inserted; cupholders that swing out of the dash, so that cups dangle above the passenger's knees; and automatic rain-sensing wipers that often got carried away and swiped far too long on dry glass. (I'm also still surprised that BMW hasn't made better use of tiny LED lights, and continues to screw a huge, hideous red plastic lamp - it looks like belongs on top of a fire truck - to the bottom of the rearview mirror, where it flashes as the alarm warning light.)

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And then there's iDrive. Journalists have made iDrive-bashing all but an Olympic sport, and I must admit that I was one of them. But this time around, I was determined to learn the creature inside and out, and while it's an uneasy truce, I made peace with the system over my week with it.

iDrive consists of an in-dash screen, with a round flat controller located in the centre console. You slide the controller to take you to one of four screens (climate, communication, entertainment and car data), spin it to navigate through the menus on the screens, and tap it to implement changes. On the 5 Series, the controller has only four lateral directions (up and down, and side to side), far simpler and more intuitive than the eight directions of the 7 Series, which also require you to move the controller diagonally. For some adjustments, iDrive is very useful: for infrequently-accessed functions, such as setting the clock, determining preferences (key fob functions, lighting, locks, etc.), adjusting the stereo equalizer or storing telephone numbers, the system is very efficient, and eliminates a myriad of buttons on the dash. You can even ask the car to transmit its information to BMW and then, if your telephone's integrated into the car's system, it will dial the local dealer and let you make an appointment for an oil change.

My remaining uneasiness with iDrive is in functions that should be immediately accessible, including vent modes and a tuning button that allows you to simply surf across the radio dial. If I want a little extra heat on my feet, I shouldn't have to page through a series of computer screens at 100 km/h to get it.

Of course, at this price level, you can expect a lot of standard features, and the 550i delivers: automatic adaptive Xenon headlamps with washers, tire pressure monitoring system, auto-dimming interior and exterior mirrors, sunroof, three-stage heated front seats, removable glovebox-mounted flashlight, integrated garage door opener, CD/MP3 player with ten speakers, and numerous safety features, including side thorax airbags, dynamic stability and traction control, and rear lights that illuminate a larger surface under heavy braking.