This case is an official Garmin accessory for the Edge 800. As you'd expect with an official product, it's snug fit to the 800 and doesn't get in the way of using the buttons or the touch-screen. It also doesn't cover the USB and microSD slots so these are still esily accessible.
It's made of thin matt-black silicone rubber which is is a bit "grippier" than the bare case - especially when wet. While the thinness of the material keeps the weight down (for those weight-conscious cyclists), I don't expect it to afford that much protection from drops etc. Equally, the touch-screen nature of the Edge 800 means that the silicone cover mostly protects the sides and back of the device. Considering that the unit should spend most of it's time clipped to your handlebars which will protect the back and sides of the device anyway, the actual level of protection offered is minimal.
Additionally, if you're interested in tracking elevation/climbing and it happens to rain, thre's a chance that the case will collect enough water to interfere with the port used by the barometer to gauge air pressure and derive altitude. This leads to very slow and inaccurate reporting of current gradient and increases the overall margin of error for altitude for the ride meaing you should probably enable Garmin's altitude correction in Garmin Connect to get an accurate total.
While it does a reasonable job of what it's supposed to do, it's not really a job worth doing - the Edge 800 is weatherproof out of the box and this case doesn't materially improve on that. If you've got a fiver to spare, my advice would be to buy some decent anti-glare scrteen protectors for your Edge 800 rather than the silicone case.