I agree with ContinentalOP's comments about security, comfort, and concealment but also point out that most IWB holsters secure with snaps or hooks that do not require you to remove your belt in order to put on or take off the holster. This is very handy when you need to go in somewhere like a school, post office, etc. that you cannot carry.

Another feature to look for is a reinforced mouth on your holster so it will not collapse and make it very difficult to reholster.

I'm very partial to the Milt Sparks VM-2.