What's it like?
Jaw-droppingly fast. We drove the B5 from Alpina's base in Buchloe to the UK - in time for its appearance at the Goodwood Festival of Speed - and with an empty stretch of autobahn in front of it, it's supercar quick.
Alpina's claim of 0-62mph in 4.7sec feels conservative - and as for the top speed, well, we saw an indicated 190mph with three and their luggage aboard, and it was still pulling strongly.
Truth is, though, that making a 5-series with this engine feel strong on a motorway was never going to be the biggest challenge faced by Alpina's engineers. More testing is the task of making it handle on poor surfaces and twisty roads, and here the firm's modifications - lower suspension, non-runflat tyres and comprehensive reprogramming of BMW's adjustable dampers - pays off.
We wouldn't say the B5 has precisely the same level of agility and subtlety as Jaguar's XFR - but it's not far off it at all, and that means it's very good indeed. Revised steering programming makes the wheel feel lighter at low speeds than a 5-series, and weightier when you're pushing on, and it's a more satisfying set-up than BMW's.
With the car in Sport or Sport+ set-up the ride is firm but still compliant (no runflats, remember, just Michelin Pilot Sport 2s, or Pilot Super Sports on the final cars) and body control is excellent.
That trick transmission works well, too; we can't think of many dual-clutch units that beat it on speed and smoothness. The engine has massive urge from beneath 2000rpm, and if you push on it makes a great Nascar-esque soundtrack. It's perfectly docile for everyday use, though; cruise along at 80mph (barely 1900rpm) and it's silent.
Should I buy one?
At around £70k, the B5 will sit above the Jaguar XFR on price, but beat it hands down on exclusivity. Even our car - production prototype number one - was beautifully finished, the spec is relatively generous by BMW standards and Alpina can also offer a personal service matched by no mass manufacturer.
Of course, you could be tempted to wait for the M5, which will offer a more hardcore experience but, almost inevitably, more compromises on ride quality to boot. But if you're more sold on a 5-series than an XF, we'd be sorely tempted to put down a deposit now. Even by Alpina's relatively high standards, this is the firm's most complete package ever.