I love these sunglasses. They are probably the ones that I wear the most. They're super comfortable and light. My wife doesn't like them and I actually think they're pretty dorky looking, but at least I can't see them while wearing them. Haha.
Changing the lenses is easy. Great for mountain biking as they don't fog easily. I've got the gold mirror lens and Smith advertises is at a brown tint, but it's actually a gray tint. It's not a very dark tint, so might not be the best option for really sunny days for those with sensitive eyes. Nice thing is if you get these shades and aren't crazy about the lens you choose, you can just buy a different lens.
For sizing reference, I have a very big head and these fit me well. I notice a lot of the reviewers of these have small heads and are happy with the fit. Must be because of how flexible the frames are. Those with bigger heads are probably best off with this version of the Parallel (as opposed to one with smaller lenses).