I get kind of bummed by the negative reviews for this product, as I have used it for about a year now, and have great results. That said, I knew what to expect, and did not expect a miracle product as some people seem to do.
It is henna, so it is messy. This is kind of a given. The mixing process is kind of a pain, but once you have done it a couple of times, it doesn't take very long. Also, unless you have extremely long, thick hair, you will NOT NEED THE WHOLE BAR. I usually start with 3 squares and use more as needed. To grate, I actually just use a sharp knife and shave the edges of the bar, square by square. I find this to be easier and less messy than using a metal grater, as the henna can get stuck in the grater. While doing this, bring water to a boil, BUT add water sparingly. I usually add about a cup of water to start, and then add more as needed. Add your water, start to stir, and then put the bowl of water into the remaining boiling water and continue stirring. This keeps your consistency good and helps to break up the lumps.
Once you have done this, you will need to let it cool for a few minutes. I usually put the bowl in the freezer for about 10 minutes. You do not want it to cool all the way, or it will get too thick. It should still be warm when you apply it to your hair.
Applying it (I think) is easy. Put on gloves and go to work. Just dip your hands in the dye and start to apply it to your scalp. Then just finger comb it into your hair, keeping the strands as small as possible throughout your head.
Wrap your hair with saran wrap and leave on for awhile. I have dark brown hair and use the caca rouge, so I usually leave it on for 5-6 hours.
Here's the thing. Henna is gross. It will drip down your neck and look like your head has diarhhea. It smells weird, and it it will stay in your hair for a couple of days before it is completely washed out. BUT, (I think) It is worth it. It makes your hair incredibly soft and shiny and actually works with your hair texture. My hair is much less frizzy after I use this dye, and my hairdresser has commented about how healthy my hair seems lately.
As for Lush's product specifically, I find that it works better than most Henna's solely because it has all of the ingredients that you might want to use in the single bar. So, while it is a little pricier, you don't have to buy as many products.
Finally, in terms of its lasting power... I usually re-do my hair about every 6 weeks. I could wait longer, but I like for my hair to be bright, and, like any semi-permanent dye, it fades.
For those who have only ever used store bought chemical dye products, this process might be a pain and a shock, but it is SO GOOD for your hair, and completely worth the pain. Also, for those who complain that the color doesn't show, this is likely because you are not leaving it on long enough. Though the Lush instructions say 1 hour, if you read elsewhere, it says anywhere from 1-6, depending on how bright you want. When you leave it on for longer, more color deposits, so the color is more likely to show. Like I said, I have really dark hair, and this shows up beautifully, so I have a hard time believing when people say it doesn't work.