I had that case on my iPhone 3GS until I upgraded to a new phone. Personally I really liked it (enough to buy more than one so I had some in different colours). It was super thin, protected my phone from scratches, and it was like it was barely there. It definitely won't protect as well as thicker cases, and I'm sure the case itself would crack or even break if dropped from more than a few feet, but hey, I always would rather have a thinner case that would need to be replaced when dropped than to have a thick case making it all bulky.
I did notice though after a few months, on the one black one I had, some of the matte finish was scratching off on the back of it from keeping it in the same pocket as my keys. Not a big deal, it wasn't even that noticeable and could have been avoided if I didn't keep it with my keys all the time, but thought I should mention it.