How useful are Geiger counters? -- Not as useful as personal radiation detectors

I understand geiger counters are actually quite inexpensive -- perhaps thirty dollars or so. What can they be used for exactly? Detetecting dangerous levels of radiation? Trying to understand and analyzie natural sources of radiation in the environment? I don't believe geiger counters actually break down the different kinds of radiation at all, so, I suppose that would be a kind of limitation on their utility. Personal Radiation Detectors break down the different kinds of radiation, and there are inexpensive electronic kits to make them.