Without wanting to be Devil advocate u often only get what you pay for.
And ure paint can be the deciding factor in who wins and losses.
But like everyone has already said it depends what you want them for, if its just for training, shooting in your back garden quality aint really too much of an issue but when it comes to playin you wanna get the best you can get - this is more often to do with the way paintballs are stored as opposed to the cost of the ball. A £25 box of paint stored in the right conditions will shoot alot better then a £35 box of paint that has been stored in the boot of a car, in a uncontrolled enviroment etc
That said the more expensive the ball the more work is put into ensure its roundness and consistency so it tends to be the better choice but like i said just how its stored makes a huge difference.
Just summat for you to think about.