This is likely one of the slimmest cases for an iPhone. It's made from understated leather with only an unobtrusive 'Sena' tag visible. The phone is designed to slip in and out. There are little holes at the bottom to hear it ring, and the top is exposed so you have access to the headphone socket.
It definitely fits the iPhone 4S, and by dint the iPhone 4. It won't accept anything else attached to the phone (i.e. no bumper).
The fitting is very tight, but it does ease up over time. There's no fear of your phone slipping out.
The case has a velvet lining to cushion your iPhone (and get read of oily face marks). In earlier versions of the case (I had a 3G version for several years), the glue joining the lining and leather at the mouth eventually split apart. Sena have fixed this with stitching around the mouth so it should last longer.
Why encase your beautifully designed iPhone in a lump of ugly plastic or rubber when you can have something simple and sleek?