speedlever, got my vertical belt case today from ebay.
The case has plenty of room for the GS4 with pretty much any case thats on the phone. Since my case is thinner then most there is a good space gap on the sides and front/back and the phone does move around some. The case would be really good for a thicker case up to .75 thick. I'm thinking of doing something to the inside where my phone doesnt move around as much as it does. The extra space does make it easier to take the phone out though.
The magnet that holds the top closed is pretty good. I dont see any issues with it coming off, but defiantly wouldnt trust turning the case horizontally 100%. At a slight angle I would, just not 100% horizontally. The case will have to be turned if sitting down though because of the size of it. This is where the horizontal case would make more sense. For $9.99 though either case is good. Just a matter of personal preference.
For anyone considering one, here is the link that speedlever originally posted for me. They also have the horizontal case. The seller gives the measurements so just measure your phone with your case on it and you'll know if it will fit or not. V078 Large Oversize Samsung Galaxy S4 Vertical Phone Case Pouch Rotating Clip | eBay