I've lost it one time. I don't know why people dislike the aluminum chassis, it seems really tough. I'd much rather have it than plastic.
It won't flex. At all. IF they do flex, they bend. I bent the shit out of my SPi's. Even if you don't bend the shit out of them, they create a dead spot in your board that doesn't allow it to flex naturally. All bindings do this to a degree, but it seems noticeably worse with that big chunk of rigid aluminum screwed to your board. Plus, they seem to fatigue the hell out of my feet. I assume it's because they're not flexing in the least which translates more strain to the feet.
No offense meant and please don't take it as such, but you're probably just not good enough to understand why some people don't care for them in terms of how they ride. I originally picked up the SPi's when I was probably about your level and had a similar thought process to yours that aluminum had to be better than plastic. As I progressed, I changed my mind.