I used powerblocks one time, but only in the store for test purposes. I think i was out shoping or something with someone and was bored. Anyway, didn't really get a great feel for them, but my instinct was that the weighting was a little strange; like i kept feeling like the handle was trying to twist in my hands.
But, sorry, can't really speak from too much experience. They do kick ass however, in how easy it is to change the weight, although, now that i think about it, i had some difficulty getting the u-shaped pin to fit in right and the elastic band holding it to the dumbbell was a little too short.
Test em out before you buy them.
-Fatty