cazzz - it works OK with me, but a lot probably depends on the colour you're aiming for. I've just got little bits of grey, and I just use a light brown colour.
Henna might be OK, but again it depends on if you are trying to cover lots of grey. I know someone who has used henna on her black hair for years and has always looked good, but now that she's getting older the henna is turning the grey hairs bright orange!
Another thought, Smow, was the kit you used ammonia-based? That really is evil stuff, and I reacted horribly the one time I used it. If it was, there are plenty of others that don't contain ammonia. I think l'Oreal Casting is ammonia free. But obviously, it depends on what chemical it was that caused your bad reaction.