I got my first kettlebells a couple of weeks ago.  I have yet to master the snatch or clean (they're actually quite technically demanding), but the swing is easier, and I've been doing a lot of it.  My karate class was in a break (we train at a YMCA and there's a break between sessions) and I haven't been practicing my kihon much at all.

We started class again and I realized something - my hand techniques were faster and crisper than they used to be.  I was getting really good hip drive into my punches and they were just flying around (fast for me, I'm not saying that I'm lightning quick or anything). 

Why?  I really think it's the swings.

To do the swing you reall lock out the hips and knees at the top, "snapping" the hip to complete the movement.  It looks a lot like the way the hip snaps into a punch or block.  In fact,  Pavel makes the analogy in more than a few places in his writings and DVD's.  So doing the swings and concentrating on that hip snap has really improved my hip drive when throwing punches.  I suspect that that same movement - full extension of the hip - is going to transfer over into greater closing speed the next time I fight, too.

I can't say yet that kettlebells are the be-all and end-all for martial artists, but I can say that the swing is a great exercise for strikers and that you can't really do them with dumbells (I've tried - it doesn't work).  Kettlebells are not the equivalent of infomercial exercise equipment.  If you can find some, try some swings and see how it feels.

Good swinging.