Got mine today. Here's a quick review:
Pro's: The Incase Perforated Snap case is a perfect fit on my 3GS. It's incredibly lightweight and flexible, and is very easy to fit. It provides good scratch protection for the back of the iPhone, and wraps around most of the bezel including the corners. The case sits around 0.5mm proud of the bezel, meaning it passes the "face down" test - put the iPhone face down on a surface and it rests on the edges of the case rather than the glass screen or bezel. The volume and mute buttons are accessible through cut-outs in the side of the case. I bought the black case, and the perforated surface has a matte finish with a slightly "rubbery feel" - very tactile. Finally, there's a good chance my 3GS might now fit the Apple Universal Dock - something I'll try on my next visit to an Apple Store.
Con's: The Perforated Snap Case leaves the top of the bezel, and the sleep button, unprotected. Likewise, the bottom of the bezel next to the speaker, microphone and dock connector is also unprotected. If you carry your iPhone around in a pocket, then over time dust and lint is likely to be trapped in the perforations. However I'm fairly confident that a quick rinse under the tap should get the case clean.
Conclusion: I really like this case for its clean, minimalist look and feel. Probably not for anyone who needs maximum protection for their iPhone, but if your 3GS leads a fairly uneventful life then it's worth considering.