Hey, I actually use a rubber case and a leather holster for my storm. I find that it works the best. I noticed (with my previous blackberries, the 8830 and the 8330) that they often slip out of your holster, especially when it's horizontal on your belt. This rubber case keeps them from sliding out, even if the leather strap with the magnet comes loose. Sure, at first, the rubber case is incredibly hard to take out of the holster, BUT I'm 100% sure that as you wear it out, it'll come out in a firm but smooth way.
Just my two cents
G'luck trying to find the best way to protect your storm
Edit: For the leather case, I make sure to use one built for the curve. for some reason, it fits better to me. A friend of mine uses a leather case for the 8800 and it works well for him as well. Both were purchased (unfortunately) at an ATT store.