Got my Spigen SGP Slim Armor case today. So I'll share my review of it:
First off, great looking case. Very clean and sleek looking. I got the "Metal Slate" color and it really compliments the "Black mist" s4 well, since in some angles the s4 has this bluish tint to it.
The case itself is of the high quality expected by Spigen. TPU case is nice and seems like it can take a real hit and the plastic cover seems to be very durable as well. Put the two together and I would say your s4 is pretty well protected. I've dropped my Nexus once with just a TPU case and that absorbed the damage real well, so if (knock on wood) I drop my s4 hopefully Spigen's design will protect it even better, especially with the inside ridges that supposedly absorb shock better.
The s4 is about as thin as it can get so any case added on you'll notice the bulk added, however, this case doesn't add too much. It's definitely thicker with it on, but this is definitely one of the slimmer cases I've ever used (though since I've only used 2 in my life I don't have much experience on the matter), point is, good enough for me. And yes, it does add enough bulk to make it feel better in your hands and definitely easier to grip compared to the sometimes slippery back cover of the s4.
A few things I did notice is that the TPU case doesn't cover all of the silver band around the front of the phone. On both the top corners there is just a bit of silver band exposed, but no biggy. I just expected it to completely cover from all the reviews. Also the plastic cover seems to be cut a bit unevenly on mine right under the "Slim Armor" logo, but again no big deal. Maybe I'm too much of a perfectionist when it comes to this sort of thing, but once you leave it on the TPU will "settle" into the right spots, at least from what I noticed in 30min with it on.
Overall I'd give this case a 9/10, only because I haven't dropped it yet and don't know how well it protects the phone against a drop. It's a real good looking case, doesn't add much bulk and actually makes the s4 easier to hold, provides a good level of protection, easy to slip on/off, and for $15 one heckuva deal.
I'll try to post pics with the phone in the case once I get another camera on hand. These photos were taken with my s4 and still don't do the color real justice. As you can see by the sunlight, the color on the back plate can really change.