I have - It makes your hair go an orangey colour, so just buy another box and put it on the day after. The colour b4 makes your hair a bit dry because of how much and for how long you have to use it, but it does get some of that dark out! Although wont be back to the natural or 100 % colour before!! Good luck :)
I used it to take red out of my hair and it went bright ginger. But I have alot of dye on my hair, I did have it dyed the day after and it was fine. It wasn't that dry after I used it but my hair is in fairly good condition.
I`ve used Decolour which is by the same man who marketed Colour B4 (there is a thread on here somewhere here about it which I replied to). My hair was a strange ginger colour after I used it but it did get the dye out. You need to be ready with a dye to cover the results though - and one lighter than the shade you are trying to achieve because your hair will take the dye much more after Colour B4
I think i'll try the stripper since I've forked out £11.99 a box for it! Haha! Kinda curious to see what colour I get anyway. Hopefully it won't be too bad for my work on Monday! =s