i had to switch because max factor is no more.. =(
but i love it!

I am extremely loyal to the Lush hair colours. I use the "Caca Marron" and it gives the loveliest auburn colour on my medium brown hair. I also love "Honey I Washed the Kids" soap, it smells sooo nice!

I've looked at the hennas many times and debated about purchasing them. Are they easy to use? How long does the color last?

They are in my opinion very easy to use, although a bit more fiddly than your bog standard box colours. You need to grate the caca block and mix it with water - the trick here is to get the right consistency. Some people find the application messy although I have to say I haven't ever found this to be the case, certainly no more so than colour in a bottle. You will need to leave the henna on for a long time - I would say a minimum of 2 hours. I usually go for around 4. The beauty here being that the colour isn't bad for your hair, so you don't have to set a timer to make sure you don't fry your hair :p
The messy part IMO is the rinsing. I am fortunate enough to have a closed in shower with good pressure, which makes it easy. If your shower has low pressure or you have a lot of hair (mine is short-ish and fine) you might want to consider soaking it for a few mins in the bath before actually showering.

The vibrancy of the red tones last at least a good month, and I find that the growth line isn't harsh at all, but that could also be because the colour I use isn't miles off my natural colour. Basically the colour lasts easily long enough for growth to be an issue rather than fading. I also find it makes my hair really glossy.
I don't think I'll ever stop using this hair dye :)

Thanks for the info timkerbelle! I think I'll have to check it out. :)