With due respect to Nathan and Kreyzhorse, I usually prefer a holster with a retention strap, concealed or not. If the owner practices sufficiently it's second nature to thumb off the strap and draw the weapon. Additionally, when carried concealed, if the owner stays aware of his surroundings and sees potential trouble brewing, an elbow can unobtrusively pop the snap loose through the cover garment for faster access if desired.
Reasons I prefer a strap....
- Better retains the gun during a "wrasslin" match on the ground.
- Worry free retention whilst mounting or descending stairs at a run.
- No embarrassing *clatter* if you trip and fall or you're pushed to the ground.
- Retention in vehicle in case of an accident. (see below)
- Much better for short-barreled guns that can "creep' out of other holsters and fall.
Some years ago, a fellow shooter was rear-ended hard at a traffic light. The front seatback failed and he found himself laying down. When he exited the car to check the other driver he found his 2" .38 missing. Since the other car's occupants looked unharmed, he looked in his car for his gun. Only when he pushed the seatback upright did he find it. The impact apparently dislodged the .38 and the car's forward motion caused it to fall into the rear footwell, landing under the seatback.