YES!!!!! to henna. I use the Light Mountain products, and at this point I have 25-50% gray hair. Henna is much healthier for hair, it nourishes it instead of giving it that fried look after 3-4 weeks. The henna for gray is a two-step process, and more of a hassle, but I found that the regular henna works just fine for gray hair; you don't need to use the kind for gray hair. I make tea and then use the tea water for the mixing process, it adds more color and seems to help the color hold. Maybe the tea is why I don't need to use the gray version any more. The only downside to henna is limited color choices. But you can mix.
Also, contrary to other opinions here, I have not found that it fades quickly at all - in fact I think it lasts longer. And if you want to keep your color rich you can do it every two or three weeks if you so desire, and it won't hurt your hair.
Just reading some of the other responses: and I'm sorry but they are wrong -- henna DOES cover gray hair, and I'm living proof of that. They may be right about using henna over previously colored hair, but the first time I used henna it was over colored hair, and nothing bad happened.
Also some people think the smell of henna is bad, but it leaves your hair after a couple of washings, and I don't see what the big deal is. Personally I don't mind the smell. And the grittiness too -- so it takes a little longer to wash out, so what. (But don't shampoo out the henna the first day you dye it, you'll lose the color.)
p.p.p.s - In response to people who say that henna never comes out -- so what? Isn't coloring your hair the whole point? And if you don't like the color you chose, you can instantly just color over it. And it does fade somewhat, so it's not entirely true that it "never comes out." Also, regular dye never comes out either and it changes the dyed part of your hair forever, so I don't know why people are making an issue of that.
Also - It does NOT take a long time, you can do it in a little as 20 minutes, but I leave it on 40 or 45, for stronger color. There's no way it takes 5 hours, per one response, I don't know what these people have been doing.
And MORE -- My hair is DARK BROWN, and it NEVER turns orange with henna, not even the gray strands. Over time the hair gets a little redder, but not unpleasantly so, and certainly less than what used to happen with regular dye.