The L'Oreal ones are all right, but avoid Garnier like the plague. I posted on here a while ago after trying to lighten my already quite blonde hair. I was almost crying. The highlights went bright orange - I'm not exaggerating.
I had to dye again over the top and now my hair is really dry and straggly.
I was proud of my thrifty ways in buying a pack of £2.95 highlights and then getting a £12 haircut, but after just catching sight of myself in a shop mirror, I think you usually get what you pay for.