When cars come to me for testing I always try to go to the same stretch of road. It’s quiet, you can see for miles in every direction, and the sinuous turns ask searching questions of any car I aim along it. Perhaps predictably this new BMW M5 Touring, tuned by the company’s legendary M division and with a 5 litre V10 engine, coped impressively. I revelled in the howl of its motor, the grip of its vast tyres and the speed of its paddle-operated gearshifts.

Reaching the end a very happy and rather impressed boy, I looked in the mirror and saw two large brown eyes gazing back at me. Yes, I’d gone down one of the best roads in the UK at what you might call a purposeful clip with a labrador in the boot.

I would make two points to mitigate the bitch in the back. First, Lola has gone fast since her earliest days and, judging by the black missile that streaks into any boot I open, is far from traumatised by her experiences.

Second, the M5 Touring may look like an estate from the outside but that’s not how it feels from within. Name another with a seven-speed paddleshift manual gearbox, an engine that revs to 8250rpm and a button on the steering wheel that will up the output of the motor from 400bhp to a truly frightening 507bhp. It even has a launch control system that is conceptually no different from that used by Nick Heidfeld to fire his BMW Formula One car off the starting grid on Sunday afternoons. Take the speed limiter off and the car is capable of 200mph. For an estate that is verging on the ludicrous.

But the really telling point is that on that particular occasion I had not been planning to test the car at all. In fact I was on my way to the beach. Hence the hound . . . and the children, wife, boogie boards, picnic and all the other family paraphernalia that happened to be on board at the time.

If you look at the motoring press, you’ll be reading a lot about this car in the coming weeks. You’ll find out exactly how fast it will go in a straight line and round a corner, but I wonder how much you’ll discover about how well it slips into everyday family life. With a body born to serve, but the heart of a racing car and the soul of a true maniac, the personality of the M5 Touring is not so much split as splintered.

Of course in Europe they’re used to all this. BMW made an estate version of the M5 saloon more than a decade ago but omitted to offer it for sale in the UK. Which, like most things that are desirable but prohibited, meant a cult sprang up around it. Among motoring hacks it became known as the best car you’d never driven. Finally I did get to slip behind the wheel of one, only once and only briefly, but it was enough to convince me at the time that, in the real world, it was the greatest car you could buy.

The same cannot be said for its descendant. The appeal of the old M5 Touring was the fact that, if you took the badges off (which was a no-cost option), the car looked hardly different from a 520i Touring which had the pulling power of an arthritic ant. The new M5 Touring has many talents, but subtlety is not among them: its body is a riot of wings, chins, spoilers and skirts.

And while it is outstandingly capable when driven as its deranged creators intended, the opportunities to do so safely are not exactly around every corner on these isles. And the rest of the time the car is not without its issues.

The ride, for instance, is firm enough to put you on permanent pothole alert. I don’t offer this as criticism, for if it were more softly sprung you would inevitably lose some of its fabulous high-speed precision. But if you are thinking about spending the best part of £70,000 it is something to bear in mind.

You need also to come to terms with the gearbox. This is another example of race-car engineering finding its way onto the public road and reminding you why race cars are kept on racetracks. As you drive, you can select any one of 11 settings for this gearbox – six in manual mode, five in automatic – which vary the speed and response of the gearshift according to your requirements. The problem is, unless you’re driving with your trousers on fire, none of them is very satisfactory.

There is nothing that would improve the M5 Touring more than the fitment of an ordinary six-speed manual gearbox.

Apart, perhaps, from a larger fuel tank. Drive the M5 quickly and 10mpg is a very real possibility. Cruise along the motorway at the same speed as everyone else and you’ll still need to plan a fuel stop into every major journey. Most users won’t put more than 250 miles between service stations, which, were this my car, would infuriate me.

And rear seats that fold flat. You’d think that providing a flat loading area was a fairly basic provision for an estate car, but not this one, nor any other 5-series Touring. The rear seats do fold forward but come to rest at a slight angle, making the loading of long and heavy items more difficult than in many smaller, cheaper and, in every other way, less well engineered estates. A small point, perhaps, but a significant one.

Also, it’s not that capacious. Not only do key rivals like the Audi S6 Avant and Mercedes-Benz E 63 AMG estate have bigger boots with the seats up or down, but so do rather more everyday machines such as the humble Ford Mondeo or Vauxhall Vectra.

Even so, I don’t feel inclined to dismiss the M5 Touring for these shortcomings. Flawed it may be, but there is still no other estate with the sense of occasion this car commands. It seems strange to set an alarm for 5am just so you can drive an estate car on deserted roads – but that’s what I found myself doing with this BMW. The point is, and with apologies to Longfellow, when it is good, it is really quite good, but when it is bad, it is brilliant.

Vital statistics

Model BMW M5 Touring

Engine type 4999cc, 10 cylinders

Power/Torque 507bhp @ 7750rpm / 384 lb ft @ 6100rpm

Transmission Seven-speed manual

Fuel/CO2 18.8mpg (combined cycle) / 361g/km

Performance 0-62mph: 4.8sec / Top speed: 155mph (limited)

Price £67,075

Verdict The maddest estate you can buy

Rating 4/5

Date of release Out now

The opposition

Model Audi S6 Avant £56,735

For Looks great, beautifully built

Against Awful ride, very thirsty

Model Mercedes E 63 AMG estate £69,085

For Huge performance, vast boot

Against Expensive, looks dowdy in this company