In Mazda's case, it's the Wankel rotary engine, for Subaru it's the boxer layout. Both are seriously compromised, but also make for sweetly characterful cars.

There was a point when those of us who care were really worried about Subaru, because the boxer engine is just too inefficient to be a success in diesel-obsessed Europe.

But it's sighs of relief all round - not only has Subaru done a diesel, it's done a boxer diesel. If you bear in mind the company only sells 50,000 cars a year in Europe, building an entirely new engine represents a serious commitment.

I've been really looking forward to this Legacy because it's something truly new, not just a rehashed marketing concept. But, as with meeting your childhood heroes, there's a fear factor - what if the engine doesn't match up to the boxer legend? What if, despite the unique layout, which does away with the need for balancer shafts and allows for a shorter crank shaft, the refinement still falls short?

Ah, stop fretting. It's awesome. It has got to be the smoothest diesel engine yet developed, to the point where you can't even feel any vibrations coming through the gearstick. Sure, there's a bit of rattle at cold and idle (another boxer characteristic), but after that, it feels just like a petrol, with perhaps a smidge less throttle adjustability.

Of course, there are criticisms, but none are about the engineering. It could be a little quicker, but the turbo's on song when you need it, and it's comfortably as fast as a diesel Mondeo.

Which brings us neatly to the crux of the matter - the Mondeo is the Legacy's spiritual squeeze, not the beloved memory of the Scoob-of-the-same-name first campaigned by Colin McRae. It's really, really good, but you need to get out of the Subaru mindset to really enjoy it. It's no rally car.

So, although the Legacy carries an awful lot of baggage, both literal and emotional, you shouldn't lose sight of the blissful underlying truth: this is as good as diesel ever gets.