I get the classes free, due to some very lucky circumstances.
Background: there is a large cluster of krav classes available on the West Coast. Generally these are quite inferior to classes taught by Israeli instructors. Unfortunately, those instructors are largely only available in New York and the Tri-State area.
I would say -- yes, it's great exercise, and yes, it's a good confidence booster.
But please, no matter how violent and practical the moves may seem, no matter how realistic and gritty the training is -- don't let it fool you into believing that you're more prepared for a fight. The more I take krav the more I realize how fragile we are and how utterly pointless violence is. The most important thing I've learned is that in a fight I would freeze up and panic.
My instructor, a woman named Amanda Wingate (krav is better when taught by women -- since they can't rely on strength, they absolutely must get the form perfect), told us all about how badass she felt when she started krav maga. She warned us against the kind of egotism that comes from training.
I was a teenage guy when I started. I really wanted to be badass, a kind of Jack Bauer/Sam Fisher/ninja figure with the power to kill in brutal and creative ways. I think most teenage guys want this, at some point or another. Do not be deceived!
I've learned counters against knives, holds, and all sorts of other hypothetical attacks. I would never use them on the street. If I were mugged I'd give up. 'Training' in a martial arts class is no match for desperation, size, and brutality.
If I thought I was in danger of being murdered or raped, I'd fight, and krav techniques might give me some edge if I could recall them -- even with muscle memory, the upper parts of the brain shut down during panic, and that makes recall tough. But in every other circumstance, krav has taught me to run or surrender wherever possible.
It's been a great experience. But it's also very scary. After learning some knife moves, or certain groundfighting techniques, I've found myself trembling simply because I've realized how very likely my own death or disfigurement would be if the situation was real.
Also, if your instructor seems like a bad person, don't let him push you too far. Some krav instructors -- one in particular, I forget his name -- suffered severe psychological trauma during Israeli military service, and they'll teach torture techniques (spitting on knife wounds) and other shady stuff.
So, yeah, sorry to wax eloquent. It is a blast, but don't let it go to your head!