Henna

Hmm really? I haven't actually used it but I looked it up loads a while ago because I'm going to start using it when I finish Uni if I need a more natural hair colour and I thought it was meant to be really good for hair.

I've never heard of henna being drying to hair - quite the opposite in fact.

I've only used henna blocks from Lush, which is more of a conditioning mask tbh but does deposit the henna - it just fades/washes off eventually.  Sort of.  A bit.
It left my hair shiny and lovely with a nice coppery sheen.  (I have naturally darkish brown hair)

For a variety of reasons, I have never used henna and probably never will. My own personal tastes aside, what really keeps me off is that it really sticks to the hair like crazy, especially the more you use it the harder it will be to get properly rid of it. Many of my friends use it and some of them had to grow it out to get rid of it, or just cut their hair. Application can be really messy as it's thick and somehow muddy, you have to mix the powder with other things (water, joghurt..this depends on you) and it takes a while to rinse it out. You have to be careful not to use metal objects when mixing the whole thing and to pay attention also when you buy it, because poor quality powders may result in a funny colour or cause damage to your hair.
Grey hair coverage isn't really the best as far as I know - I know of people that used a "basic" red powder and their greys only got orange, not red as the rest. But that may depend on the colour...

Also, if you dye with henna you can't really switch to chemical hair dyes (unless you stop doing henna and wait for some months... I've been told at least 6). If you want to change your haircolor often, it might not be the best choice.

Aye I've heard/read that the only way to truly remove henna is with scissors!
And it is incredibly messy and weird and a real old pain in the bum to apply, smells funny, and takes ages to rinse out.

Luckily I didn't get any weird/undesirable effects bleaching my hair that I'd previously used Lush henna on, but it had been a very long time (over a year I think) since I'd last used it - but even so, I did still have that henna effect on my ends when compared to my virgin roots.
So yeah, henna can be extremely permanent.

I can't comment on grey coverage from experience but I seem to remember the instructions saying something like grey hairs will still take the henna but because they're lighter than the rest of the hair they might shine out really bright copper/red.

Yes Lush henna is a bit easier to use than powders, but the colour range is a little limited as far as I know. Well, all henna is quite a limit as it comes to colour...

Oh and if your hair is curly, you may notice that henna weighs your curls down, making them look more like waves.