I've got one and you can ride off road reasonably with the 240 lumens they chuck out. The biggest plus point is that they take AA batteries, so you can carry spares. The bar clamp works really well (it does on my O/S bars anyway).
Having said that they're about ninety quid if I remeber correctly, and the DX light (900 lumens) I use now completely blows it out of the water, at almost half the price.
I still carry the Hope as a back-up light, and it's handy for road use, and makes a great torch too. And it's a lovely bit of machined metal, which is nice, but knowing what I do now, I wouldn't have bothered buying it last year, it doesn't compare to the power of the DX.