The easiest way for me to project it (and don't take this to mean I'm a professional photographer or complete HDR hater) is this: HDR is to photography as rice is to modified car.

You can put a body kit on a car and make it look great, but 90% of the time when you see a body kit on a car it's unpainted on a primer 240 or Civic. HDR goes the same way in that you can have an HDR pic that looks great or you can have an HDR pic that looks like 90% of the HDR pics you'll see. Make sense?

In order to fully understand when HDR should be used, you have to understand the concept of dynamic range. If a scene's dynamic range is larger than what a camera's sensor can capture then by all means you should use HDR. If you're just doing it for effect, then it is effectivley just camera rice IMO - it can look good but chances are you're just trying to turn an average picture into something that it's not.

Call me a purist, call me an HDR hater, call me whatever you want... but that's my view on it.