Got my White Slim Armor in today. Liking it quite a bit so far. First impressions: great build quality, phone fits snug. Love the style, the biggest reason I bought it. Case adds a little more bulk than I expected, but to me this a good thing because while naked the phone feels a little too fragile in my hands. Regardless, it's still quite thin. With the case on the phone feels extremely solid and at least to me gives a better grip.
The volume/power buttons work well for what they're given. First let me say that I don't like the S4 buttons. IMO they're not raised high enough where it could be difficult to press the power button, and their clickiness just doesn't satisfy me. This is where the case improves on the buttons, their profile is raised just enough (not too much) where you can find them with ease. I noticed though you don't get tactile feedback unless you press fairly hard. If you do you get a nice click. Though sometimes I press the power button semi-lighly, get no feedback, and the button still activates. I can't blame the case on that though.
If there is one thing I don't like, it's that sometimes you can VERY slightly see the "metallic" trim on the phone... for OCD people or perfectionists I can see this being a problem. I would prefer a "tight" fit as I think this would completely hide the trim, but I can see why they stayed with "snug" as it was extremely easy to put the case on the phone. That doesn't mean "snug" is bad at all though... I tried to shift the phone around the case and it yielded no movement at all. Hopefully it stays that way.
And here are some quickie low quality pics taken in a mirror because I don't have a real camera
(WHITE Slim Armor case on a BLACK GS4)
EDIT: One
important detail I left out... the back of the case has the slightly raised plastic piece over the speaker, meaning that sound is NOT muffled when laying the phone down on a hard, flat surface. I believe the Spigen Neo Hybrid does
not have this. A win for the Slim Armor if you're debating on getting it vs. the Neo Hybrid.