If I’d punched every person I’ve heard say “There’s no such thing as a bad car any more” I would have a jab worthy of a deputy prime minister. As phrases go it is one of the laziest, most stupid and most dangerous in this business.

I’ve heard it from the car-buying public, the car manufacturers and the allegedly critical motoring press, and every time I’ve seen, heard and smelt a rat.

It’s true that cars are more capable today than ever before, but achievements are relative and as they improve so we should raise our expectations. They might rarely break down these days but that merely makes them reliable and should not be regarded as a point in a car’s favour. You wouldn’t praise your kettle because it was capable of boiling water; such things should be givens. Regardless of its abilities, I would argue that the worst car in any given class is by definition a bad car.

This approach is no respecter of price, brand or badge. Apply it and you’ll see that all manufacturers can and do build truly bad cars.

Even Lexus. How could a company boasting some of the most envied (and deserved) quality credentials of any marque build a truly bad car? I know only that it has and this is it. It’s called the SC430, it costs £50,850 and it’s a complete mutt.

This was meant to be Lexus’s answer to the Mercedes SL sports car and coming from those responsible for the next best limousine after the Mercedes S-class (at least until the Audi A8 goes on sale), it’s not unreasonable to expect the SC430 to show commensurate strength against the SL. When it was first released to the press last year, however, the reception it received was less than welcoming. I, on the other hand, said nothing, on account of the fact that until now I’d never driven one. At the time I did suggest to a magazine that had commissioned me to write a comparison of five luxury convertibles that the SC430 should be included but I was stopped mid-flow by a snort of derision. To say it considered that the SC430 would not have had much to contribute to the debate is putting it mildly.

This, then, is the new SC430, with modifications meant to put this sad state of affairs in the past. They centre on the car’s suspension in general and its spring and damper rates specifically, which have been blamed for the SC430’s apparently hitherto wooden handling and awful ride. The good news is the second-generation SC430 goes broadly where you point it (even if the steering is almost devoid of feel) while serving up a ride that’s poor but hardly appalling. The bad news is that this is not nearly good enough.

Compared with Mercedes’s SL 500 (£67,790) the SC430 is almost farcically off the pace. The SL is fun to drive, the SC is not. The SL’s ride is taut, controlled and fluent, the SC’s is flabby, undisciplined and, should you hit a pothole, decidedly jerky. Both have sophisticated steel folding roofs, but while the SL turns a beautiful convertible into a classic coupé, the SC is equally and irredeemably ugly, whatever you do with the roof. If the Mercedes is considerably more expensive it more than deserves to be.

For something more broadly comparable on price, try the £42,370 BMW M3 convertible. Contrary to what Jeremy would have you believe, it is a brilliant car (if you skip the optional SMG sequential transmission) and probably never looked a bigger bargain than when parked next to the SC430.

If you don’t mind the way it looks and all you wish to do is cruise on smooth roads then the SC430 has its place — and this might explain why it does rather better in America than here — but otherwise it provides startlingly few reasons to tempt.

50 grand out of your pocket. I guess it’s such a rare sight on our roads that it’s guaranteed a kind of exclusivity but few cars I’ve driven in the past year have more deserved to be rare than this.

Yet it is not without merit. It’s better built than an SL and the engine/gearbox combination is both smooth and responsive. With 282bhp from its 4.3 litre V8 it’s quick enough, too. In addition, it’s extremely well equipped and I’d be staggered if anything at all went wrong in its first 100,000 miles.

But the car remains one of the few on the road even more pointless than a Ford Fusion: it’s too dull and flawed to be a driver’s car and too ugly to cut it on the automotive catwalk. When your rivals include cars as beautiful as the Mercedes SL, Maserati Spyder and Jaguar XK8 convertible, being at best quirkily different just is not enough.

The easy conclusion is to say the SC430 is not a bad car, merely that its opponents are better. But I feel disinclined to let it off so lightly. The fact that I’d rather have any rival would make it bad in this book regardless of its actual abilities. And when those abilities are so limited that there are convertibles costing £20,000 less that eclipse it either to look at or to drive (Audi TT Roadster and Porsche Boxster), its real standing is revealed.

Over the years Lexus has produced cars that have tended to vary in ability from competent (IS200, RX300) to downright astonishing (LS400, LS430). But the SC430 is none of these: it is a poor car, pure and simple. The only astonishing thing about it is that Lexus let it be built. Geniuses, it seems, still have their off days.

Vital statistics
Model Lexus SC430
Engine type V8 4293cc
Power 282bhp @ 5600rpm
Torque 309lb ft @ 3500rpm
Transmission Five-speed automatic
Fuel 23.5mpg (combined)
CO2 287g/km
Co car tax £7,097 for a higher-rate taxpayer
Acceleration 0 to 62mph: 6.4sec
Top speed 155mph
Price £50,850
Verdict The first truly undesirable Lexus