My wife has this one: http://www.crankbrothers.com/product/view/140
Though technically we both use it on our commute - since we commute together.
I have the M10 model for myself - it's kinda heavy, but it has the other heads I wanted. I don't always bring it with me on rides.

I'll agree that there isn't much you need it for on a regular ride unless something incredibly drastic happens. I only ever needed one once, when for some reason my brake caliper got loose. Borrowed one from someone else on the MUP.