Had these on for about 2.5K miles now, and have to at least partly agree with what photographicsfaris has written.

My perception/opinion on the Heidenaus now, is this:

Tarmac: better than TKC's in the dry, about on a par in the wet.

Off-road, dry (ie gravel, shale): No complaints. Enough grip to inspire confidence to do whatever I want to do. No better than TKC's though.

Off-road, wet (ie mud!): Not as good as I'd hoped. Tread grooves, although deep, are quite narrow and rapidly fill with mud/clay. Slidiness ensues. Not as good as TKC's in this environment.

At 2.5K miles, the rear is about half worn.

Overall opinion is 'not bad performance for a good price', but I'll be putting TKC's on next time.