Alpina's are what they are....a great mountain trail bike.
They've got a mix of features like Dave says, light weight, manouverable, and torquey, but with a wide ratio gearbox so you can move a bit quicker than a proper trials bike. They come in 250cc or 350cc power.
Norm Frasers sold a lot of them in Newcastle in their day, I couldn't even get one as they'd all gone at the time....had to buy a Mk5 Matador instead.