Just started using these more regularly and am very impressed. Very light and minimal feeling, but with enough cushioning and support to not mess up your calves if you're not really ready for properly minimal trainers.
Terrible on anything muddy and slippy I'd say, but great for hard-packed trails or dry conditions.
Agree with earlier posts that Trail Rocs would have to be amazing to be worth the extra money, although having a quick look on the internet I can't see loads of stock around. Maybe there's an updated version coming?