This case is very large.  One of my regular-sized glasses cases fits inside of it, but when it is inside there is no room for anything else in the case.  The strap on the case allows me to hang it vertically on my belt (yes, it is not fashionable, but my prescription sunglasses cost a lot of money), and the metal snap on the strap lets me remove the case without having to remove my belt.  I could clip the case to a belt loop with the snap hook, if I chose to, but that really looks unfashionable.  My aviator-style sunglasses - with sunglasses straps attached - fit inside of the case very tightly.  Without the sunglasses straps, my sunglasses do move around inside a little, but the foam interior doesn't scratch them (my sunglasses are made of glass, though).  One thing about this case that I do not like, is that it pops open when the zipper is opened.  So, make sure you do not open the case unless you are holding it on a level plane, or your glasses may fall out. Otherwise I really like it.

Update 2014 - The zipper came off the track about 2 years ago.  It was impossible to get it back on without cutting the track, but after I did so, and reattached the zipper, I closed off the track with epoxy, and the zipper works fine again.  The case does not open as wide as before, but it also doesn't pop open the way it used to.  I can live with it the way it is now.  I also took the clip off and replaced it with a carabiner.  Pretty sure I spelled carabiner wrong, but you know what I mean.  It is very nerdy looking with the new clip, but it is much more secure.