This was a blackburn voyager 33 -

I had it fastened to my handle bars. I first fastened it upside down, because if it was on top of the handle bars I wouldn't be able to store my bike upside down (my preferred method, don't have hooks). The strap stayed on the handle bars but the light slipped out of its cradle. I figured it did that cause (duh) it was upside down, so I unfastened the thing and resecured it right side up, again on the handle bars, and this time it fell off even faster.

I am thinking I will tape the heck out of it to the straps. Either that or I'm just missing something obvious about how it's supposed to click in.

As for commuting as exercise, that's totally my thinking behind this. Though I'll take all the other benefits too