I got a cheap one (less than £20) from Amazon a while ago and it made my gums bleed. Should have had it on a lower setting at first to get used to it according to some comments about it.
Did find water went everywhere except my mouth. Sure there's a knack to it though as it was like using a Karcher someone mentioned earlier

Must admit though, it did get a lot of bits out from between my teeth that my Sonicare brush misses.
Bloody big things as well due to the watertank etc.