Having had the good fortune of queueing for two hours in an O2 store recently on the off-chance I could grab one of a small number of available iPhone 4's and being successful, I've been testing the Incipio dermasSHOT case on it since.
Being a long-term advocate of Incipio cases, and the fact that Incipio were one of the first to make their iPhone 4 cases available, picking my case of choice was relatively easy. As usual the dermaSHOT comes with a screen protector, and the Incipio ones are a good example, being slightly thicker therefore giving good protection and making fitting slightly easier than with some of its wafer-thin counterparts. The case itself is made of high density silicon which has a nice soft velvety feel. The fit is very good, and gives edge protection all around, as well as fully covering the back. The other thing I like about this case is that it gives a slight ridge around the front screen , which gives you a little bit of protection should you accidentally drop it face down (very important when you're looking at £140 to replace the glass through Apple).
In terms of size, this does add a bit of bulk to your new phone. In fact put it side by side next to an iPhone 3G/3GS (no case) and you'll see its pretty much exactly the same thickness. Fits nicely in a pocket, so I'm not concerned by the extra size bearing in mind the protection its giving my new purchase!
All in all, another quality product from Incipio - definitely recommended.