Most of what you hear about any art is hype, but JKD does have some solid principles. If you go in expecting it to be the be all and end all of reality fighting then you'll be dissapointed. But a good JKD school will teach you some good skills and concepts that you might be able to apply if you get in a fight.

I believe JKD varies more widely than most martial arts because it's not really intended to be a rigid set of excercises and such. Therefore the quality of the JKD will be even more dependant on the instructor than for most arts.

I don't do JKD however, so an actual practitioner may be able to tell you more.