Anyone tried it? I'm running 646s right now, and they're cool, but weigh about as much as a small moon, and I'm good enough at getting clipped in and out of them fast that I don't really ever end up on them not clipped in. Seems like I can probably save myself quite a bit of weight with something like 540s. Only real concern is how they're going to hold them up to getting smashed into rocks all the time. Thoughts?