I used Garnier Olia, which was really easy to use - just a case of mixing the developer and colourant in the provided pot, giving it a shake and applying it. There were lots of nice touches which made it easier, such as a pop-out hole in the box to stand the applicator bottle in so it didn't spill, and the pointed spout on the applicator which made it a lot more simple to put on.
My hair, afterwards, is much more uniform in colour - the very bright highlights are now the colour they should be, and the dark section at the bottom of my hair has lifted. I have done a video to show my new hair.
My hair feels surprisingly manageable and was so easy to straighten afterwards - I'd been worried that the condition of my hair would deteriorate, but in actually feels loads better, like I'd had a hair treatment in!