I've used it a couple of times, it works okay. It's a bit easier to work with than henna powder, and if the hay smell of straight henna annoys you then the Lush an be more pleasant. The main downside is that because it's in a block and exposed to air, it oxidizes and therefore often doesn't give as brilliant a colour as body-art-quality henna.

I've used Lush henna the past few times I've colored my hair. Here's my experience:

First, to address your question about Lush henna over dyed hair, I had been using store-bought box dye before I tried the Lush henna, and my hair didn't have any negative side effects when I switched.

I agree that the Lush bars do not result in as vibrant a color as body art quality henna (or store bought dye). For me that was actually a positive because I want to let my natural color come through and just color the grays. The Lush bar gave my grays kind of a coppery tone so they look like natural highlights, and the condition of my hair overall was definitely better after using the henna bar (softer, less flyways).

So far so good, right? Well, not exactly. Here's what I didn't like about the Lush henna. I used the Caca Marron bar. One of the ingredients is coffee grounds, and I've found that the smell is just too much for me to take. I feel like my hair smells like rancid coffee for about a week after I use it. It's nasty. Also, preparing the henna from their bar doesn't seem to save any time compared to making a mix with body art quality henna. To really get the consistency right with the Lush bar I've found I have to cut the amount I need off the bar and then chop that part up into very thin slices, almost like shavings from a chocolate bar. Then I put the shavings into a double boiler on the stove. As it begins to melt I start mixing in hot water. Once everything's melted I take it off the stove and add olive oil (otherwise it starts drying out before I've finished applying it to my hair). From there it's the same as any henna mix: apply it to my hair, wrap in plastic wrap and wash it out in 3 or 4 hours.

I've almost finished using the Lush bar I bought, and once it's gone I plan to find a good recipe on hennaforhair.com and start using body art quality henna instead. I tried the Lush bar thinking it would be easier, but it really isn't.

thanks for all the feedback! i think i am actually immune to the smell of rancid coffee, i smell like it every day from work, haha! I am interested in trying it but my natural hair color is much lighter than the color it is now and i was affraid that the color wouldnt come out even (ie brighter at the roots and darker everywhere else)