I bought and am using the L-bracket. I haven't tried using a cable release with the camera vertical but i don't see that it'd be a problem: access to the ports is pretty much as it is with the other L-brackets. When I bought this one it was cheaper than the rest (the prices now see about equal), and it's also lighter. I have no complaints about it at all.
I want to thank everyone for their comments. I just received my Markins L-bracket for my 5DMarkIII, and it fits beautifully.
I thought I might have some problems with my 3rd party cable release fitting in but it works fine, both in horizontal and vertical positions, though you'd definitely want to plug in the cable release before you switch to vertical position.
I've not used an L-bracket before, so I have nothing to compare it to, but it seems fairly unobtrusive (though visible at bottom left where it doesn't hug the body), and pretty light, which is important to me, since I do a lot of non-tripod shooting.
