I've got one of the X-stoves. I don't have that much trouble with the locking mechanism (except for one gas cannister that had a dent in the collar) but I did have a problem with the gas tube leading to the stove. It was one of the earlier models and Coleman has since redesigned the tube with a heavier duty wire braid to protect it. These are the only butane/propane stoves that I would use below freezing. They work well at altitude compared to other gas canister stoves.

But as with most canister stoves, it is a good idea to preheat them by putting them next to your body for a couple of minutes to improve performance in cool conditions. I've heard that putting the cannister into another small container with water will also help to prevent it from cooling off and therefore keep the pressure up.