We had a thread like this a few months ago, I think. It stayed surprisingly good-natured, given the tendency of all martial artists to declare their style supreme and all other styles rank imitation after about ten minutes of conversation.
I've studied Kenpo Karate for the past 11 or so years. Shoot, coming up on 12 now. I'm one of two 5th-degree black belts in the school, the two of us ranking only under the two instructors. It's not a terribly fast-promoting art or school -- I advanced faster than normal because a) I've got a really good memory, and b) I was a geek who had no life except doing BBSes and practicing karate.

I'm not saying that to brag -- I just don't wanna make it sound like my school gives people their black belts on the third week of class.
Kenpo has helped me a bunch. I've never had to use it on the street, but the REASON I've never had to use it is because I knew that I could break elbows, knees, or necks if I had to, and the projection of self-confidence was an effective deterrant to a few would-be threats. Beyond that, it's kept me in good shape even as my youthful metabolism slows down, helped me focus my mind and get my pscyhological stuff in better order, and taught me a ton about humility and self-confidence.
EDIT: As far as street practicality, it really depends on the style, and the school you're studying at. Some schools look all artsy and flowery and the motions are soft and beautiful, but when you get to a certain degree your instructor says, "Hey, by the way, in this move here, we're actually raking the groin, elbowing them in the throat, and putting a thumb into their eye." My style starts out pretty pragmatic and then gets more graceful (while still staying street-smart) as time goes on.
Gracie Jujitsu is certainly an excellent street-defense style, since they currently have pretty good control over who gets to teach. Tae Kwon Do and Ninjitsu (or Bujinkan or Hoshinjitsu or whatever) get a bad rep because a bunch of knockoffs and frustrated students spun out their own schools without a real understanding of the deeper concepts, and tried to fill in the gaps with their own goofy philosophies. REAL versions of either of those styles work just fine on the street.
-Tacky