I don't like small-of-the-back holsters because they're difficult to draw from, but I do like the useful pocket holster--a convenient way to pack small pocket autos and revolvers. A good pocket holster should be easy to draw from and stay in your pocket when you draw your pistol.
Other specialty holsters such as fanny packs, belly bands and undershirts with integral holsters are very specialized items. I use them all on occasion, but I limit their use to running, biking and hiking.
Ankle holsters are best left for last-ditch back-up guns. They are difficult to reach and not all that secure. Plus they expose your pistol to too much dust and grime, which are tough on semiautos.
Basic Rules: Concealed Carry Rigs
A good holster must be made for your pistol. Generic, one-size-fits-many models don't do a good job of retaining your pistol and are generally not very comfortable.
A good holster should be molded tight enough to retain your pistol without straps or tension screws, and it should cover the trigger guard. It should also have a reinforced mouth that keeps the empty holster open for easy one-handed re-holstering. The holster's belt loops or slots must match your belt so the holster stays put for all-day comfort and a sure, speedy draw.