Chaps and 'esses, a word to the wise. In the run-up to the London marathon we are seeing loads of foot problems due to people running on tarmac in this type of shoe. The lesson is you need to feed your mileage in VERY carefully and stop immediately if you get any kind of discomfort and restart at a lower level.
These things will wreck your feet if used injudiciously...much like anything else really.