The henna is messy, gritty/greasy and smells odd. Look past those things and the bars work really well, I used to use the marron one and it was nice. I think I used half or a third of a whole bar for my thick shoulder length hair, it depends on your hair type and length how much to use. Some people leave it on all day for a stronger effect, I would try for at least 1-4 hours with clingfilm on my head (heat for a redder brown colour). Others add different things/scents to the mix, have a google.
However, be aware of a few things:
Henna coats the hair rather than lifting the cuticle to deposit colour there. As a result, eventually after 6-12 months of full head applications, your ends will have a lot more colour on than the roots and be a totally different shade/condition. It's advisable to try and apply future henna only to the roots and just do a whole-hair colour a couple of times a year.
Some hair doesn't get on with henna, some loves it. Mine didn't like it, it became brittle and straw like :( I liked the colour but henna almost destroyed my hair, I was really disappointed (this was all with the Lush stuff). You won't know which you are until you try it.
And last but not least, following on from the above, you can't get rid of the colour once it's there. You can try chemicals dyes over it but most chemical dyes advise against this on the packet. The only way to completely remove the henna from your hair once it's on there is to grow and or cut it out.
That said, if it works you for, brilliant! Lush henna is fairly good quality, though I've heard good things about the body art quality henna available from Hennaboy too. Never use cheap poor quality henna powder from health stores etc, it's mixed with other things which can turn your hair green if you ever use chemical dyes over the top! 
Only ever use pure top quality henna. The Lush stuff is mixed with cooca butter and coffee or herbs, but these are alright. I dyed over Lush henna no problem.
Good luck :)