As with all similar concepts, I think it can be handy in forming a personal ethic as well as coping mechanism, but as something in and of itself I would be skeptical. That is to say that I don't believe in it specifically but I believe in its potential for usage (kinda like the way I regard religion).
Often, what goes around does really come around- there are any number of ways of explaining this general principle in specific scenarios. For example, a risk-taker who deliberately defies death habitually will be more likely to die during his act of death-defying. The difference is that while somebody who believes in karma might say, if the nut did buy the farm- "well that's karma for you" as in his death-defying actions caused his death. However I would disagree, because this is an incorrect application of the concept of causation. In this example, a probablistic model might be more handy here: suppose that there is a certain probability (it doesn't even have to be consistent) that during the act the nut will die. While the probability per event does not actually necessarily increase (that'd be like the karma model above), after an increasing number of events, the probability that the event of dying will have taken place either at that event or beforehand will increase. The difference between the models is that I consider coincidences to be just the same kind of event as any other: they can be inexplicable or extraordinary events but I need not call them miracles or a divine act, especially as there could be many contingencies at play that we are not even aware of.
Where karma is most useful, to me, is in describing interpersonal relationships. If you're an asshole, you can reasonably expect (but not necessarily within your lifetime) that people will get sick of it at some point. But karma to me seems to suggest that life is somehow fairer that it really is, which is to say, it isn't at all, nor does it purport to be IMO.