I'm a huge fan of crystal cases for the PSP, mostly because it reveals all of the PSP while offering great protection. On top of that, it's really cheap. However, I haven't come across a good crystal case for the new Slims. I ordered one off Dealextreme and it was terrible.
A couple of weeks ago, I decided to invest $13 (shipping included) in a HORI Crystal Case 2 from play-asia.com. I just got it today and it was 100% authentic, straight from Japan. And I have to say, I'm quite impressed. The quality was the same if not better than the ones stateside for the PSP phat. There were a couple of minor scratches on the screen upon opening it, but they're not visible unless it's under a strong light. The case fits perfectly and doesn't add much thickness (not as thick as the PSP 1000). Even with some minor flaws, I have to say I'm in love with the case (so much I got a screen protector for the case so it wouldn't scratch). So if you like to show your entire PSP while giving it protection I recommend this case very much (I have a ceramic, so it's nice to show off the white).