I had my Blackberry 8830 in one of those snap-together plastic cases. And, as far as protection goes, it worked well. In fact, it was kind of like having my BB in a tank.
But, I didn't like it for a few reasons:
(1) It made my BB look like it was in a tank and added a little too much bulk for my liking.
(2) Battery pull is impossible. I needed a screwdriver to pry open the case, no easy task. And after doing it a few times, one of the tabs broke.
(3) No protection for the keyboard/screen/trackball.
You see, I keep my BB in my pocket. While the included holster is nice, I simply cannot bring myself to put something like this on my belt.
So, I went looking for a slip case. Something that was thin, provided a little cushioning to give a little protection, and would cover as much of my BB as possible.
Most slip cases are really holsters, with clips on the back. This adds thickness that I don't want, since I'm not using the clip.
Finally, I settled on the Leather Pocket, available from Shopblackberry.com. I found it listed a few other places, but many seemed out of stock. Plus, I had a nice Shopblackberry.com coupon.
BlackBerry Accessories Store - Product Detail
What I got was a thin, leather, slip case. On one side is the "Blackberry" name and logo. The other, the logo. This is an authorized by RIM item.
My 8830 fits perfectly inside. One side of the case has a groove that fits the trackball, so the BB can only really be inserted one way.
The case includes the holster magnet, so sliding my 8830 into the case puts it to "sleep mode" (or whatever it's called) and activates the holster profiles.
I covers all of my 8830 with the exception of the speaker, top, and a area of the back that's the same size as the speaker. These are finger holes, allowing you to pull out the BB. The LED is easily visible. If I push the power button, I can kind of see the area of the screen where it shows notifications (e.g., mail).
What I like:
(1) It's thin. Short of those plastic wraps, this is the thinnest case you'll find.
(2) It looks nice, being a dark brown (almost black) leather. It just so happens to match my wallet.
(3) The holster magnet. Although I've yet to see why I'd have different profiles for in/out of holster.
(4) LED is visible.
(5) It covers almost all of the BB, giving reasonable protection.
What I don't like:
(1) The price. At $30, it's not the cheapest case around. And shopblackberry.com's shipping costs are (in my opinion) high. Luckily, I had a 15% off coupon.
(2) It requires two hands to remove the BB: one to hold the case and another to pull the BB out. This can be inconvenient/slow, particularly when driving, to see who's calling.
(3) No protection for the speaker/earpiece. Lint from my pocket can still get in there.
(4) No protection for the power and mute/standby buttons. They can get pressed accidentally.
(5) If I don't squeeze the sides when I put out the BB, the case may rub on the trackball or keyboard.
(6) Because it's totally seperate from the BB, one the BB is out, I gotta put the case somewhere.
All-in-all, however, I find it finds my needs. If there was someway to pull down a flap to see the screen (to see who's calling and to activate the speakerphone), it'd be pretty near perfect.
If you're someone that hates the holster, wants something that looks professional, and wants to keep their BB relatively clean and protected, this should work.