We have had our Dyson Animal Ball for over 3 years now and love it. It does a great job on carpet and hardwood/tile. If you get the "kick back" while on hardwood/tile, turn off the brushes, this will eliminate the problem. On hardwood/tile I always leave the brushes off unless its not getting all the dirt up.
I will say this though... a couple of weeks ago, we had a Kirby salesman come to our home and he had us vaccum our carpet and then he vaccumed with the Kirby and it still got up more dirt... so I don't know. We are still loyal to our Dyson (and ticked off the salesman lol) but the Kirby was very enticing.