Nope, they are not unliftable. They come in a variety of weights. I have a set that starts at 8kg/18lbs and go in 4lb increments. Plus, a lot of the moves are based on the mechanics of the swing, so you have the momentum of the bell, plus the thrust of your hips, that aid in the lifting (and creates the whole body workout, because you're using every muscle).
Here's an example of the swing:
Kettlebell Swing - YouTube
This instructor does a great job of teaching the foundations. Check out her other videos if you're interested in more moves, like the snatch (I kid you not).
I'm a huge KB fan, if you can't tell. Training for a competition in November.