I'll have to keep an eye on this post as well, since our daughter has asked for a rock tumbler this Christmas (it was the one gift she mentioned to the jolly fat man when we went to get Santa photos this past weekend) and we've been researching all the options. We're leaning toward a vibrating vs rotating, due to the amount of time it takes (a couple days vs multiple weeks - definite plus for the slightly less patient short folk), the noise level (vibrating are apparently less noisy), and final outcome (vibrating keeps the general shape of the rock, just polishes it, while a rotating will round out the edges and give you the same shape for all of them. both are lovely, but I kinda like the idea of keeping the unique shape to the rock.)
With all the time we spend on the beaches over the Summer, we've amassed quite a collection of fun little stones that I'm interested to see with a new sparkle and shine. I'm shamelessly excited about this kind of stuff, so it's fun to see my daughter taking an interest as well.