I just wrote a huge response to this. Then I shortened it.
1) Kettlebells are different than dumbbells. Many things can be done with both, some things aren't as effective with a dumbbell, however, and that's the truth. There is marketing hype, yes. There is also difference. I would MUCH prefer to swing a heavy kettlebell between my legs than a big wide dumbbell, for obvious reasons.
2) You will never be able to load your legs for strength with kettlebells like you will with a barbell. Ever. Tons of guys (maybe one or two girls) here squat over 300lbs. I challenge you to find a 300lb kettlebell, and then I challenge you to afford it, and then I challenge you to figure out how to squat with it.
3) Gradual loading with KBs is a function of your wallet. With a barbell it's easy to afford a couple 2.5 or 5lb plates in addition to your 10s, 25s, and 45s. With KBs you will be making the jump to the next weighted bell by first increasing the number of reps you can do with the lower. As you can imagine, this makes it all but impossible to stay with a strict 3RM building plan, for instance.
4) What are you aiming for? Barbell > KB for focused strength work and load. All day. However, consider your resources. If I had enough cash to buy a bench for preacher curls, I would have stacked up my gym with kettlebells first for their conditioning, shoulder building/stability, and core benefits.
It's all about what you want. This forum is largely focused on the pursuit of more strength or more power. There's a reason those goals are represented largely by powerlifting or olympic lifting competition. However, many guys here are amateur or pro martial artists, fighters, health nuts, strongmen, athletes, rugby players (holla,) current or ex-military, hardened street criminals, etc... and could benefit greatly from kettlebell work.
