UPDATE: Now it's four years since this post. I'm still on the same set of XM1+ tyres. I have now done 80,000km on them (including 15,000km off-road & S.A gibber) and they look like I'll get another 10,000kms before needing replacement. As far as $ per km goes they are unbeatable. Never had a flat, a tear, a leak or a blowout so I'd find it hard to switch to another tyre when I can trust these with my life.
Of course something more chunky and less road-based would be great but the cost goes up and it seems the durability goes down (unless anyone here can report a chunkier A/T tyre with similar mileage durability).