I've had transitions for regular glasses (not sunglasses), and they were too slow for me, taking half a minute or more to fully transition. In dappled light (sparsely spaced trees) they would be useless.

For mountain biking, I wear sunglasses with swappable lenses, and keep spare lenses in my Camelbak. In total I have: clear (for when I accidentally stayed out too late or a storm front moved in), mirrored bronze (for bright sun and no shade), and my most commonly used set, which are pinkish, just slightly reflective, with blue hilights. I can't find a perfect match right now on the rudy project site, although I suspect it's what they call "racing red". They seem to span very nicely.

I'm in Colorado, so in places with less severe sun, I might not feel the mirrored lenses were necessary.