I've been using it for about five years. I get mine from Mehandi.com. I have a lot of hair, but it's very fine. The henna coats it and makes the hair shafts thicker and stronger. My hair is nearly black, with a lot of gray. It turns the gray dark-but-bright red, and the nearly-black gets a deep auburn color. Since I've been using it, my hair is as long as it ever has been - down past my waist.
As for tips:
I always henna over clean hair. I let the henna set for about 24 hours before using it. Let it sit someplace warm. I leave mine in the microwave, partly covered. I mix it with yogurt and lemon juice. Too much lemon juice dries my hair out too much. I sometimes add a little bit of honey, powdered cloves, strong Red Zinger tea, or hibiscus flower tea to the mix to bring out the reds. There are lots of recipes out there. Just do a search. As long as it's food and you're not allergic, it ought to be safe. Patch test if you are concerned.
I sometimes leave it in for as long as overnight and wash it out in the morning. If not overnight, 4-6 hours or longer. I usually cover it with plastic wrap, and plastic grocery bags. Sometimes I heat my head a bit with a dryer. Heat seems to help bring the color out. Henna is messy to apply, and my husband often helps me get it down to the roots. He wears gloves, because it stains his hands. Make sure you put lotion or use cotton strips/tissue on your ears and forehead to avoid stains from drips. Wear a shirt you don't mind staining, and if you leave it in overnight, be careful of your sheets.
I always buy from the same source, because some overseas sources use heavy metals which you do not want in your body. I would not buy from an Asian market unless I knew about the brand and had read up on it extensively online. It just isn't worth it. For the same reason, strenuously avoid the ones that say you can dye in just a short time. They are formulated more like regular hair dye, and who knows what is in them, since they only have some henna.
I wash the henna out with conditioner, not shampoo. Cheap White Rain or whatever else is on sale from the drug store. It takes a long time to wash out, and the conditioner makes the hair slippery and harder to tangle. Use liberally. I use up to half a bottle at the time.
You won't see all of the color for a day or two, because it has to oxidize in the hair. It seems to darken and deepen nicely after about a day and a half.
The only drawbacks I can think of are that it is hard to immediately re-dye henna'ed hair with something else if you don't like it. The way it coats the shaft makes it hard for the hair to accept new colors. In my experience, the henna eventually fades enough to redye after eight weeks or so, so it's not a forever sort of thing if you really don't like it. Honestly, though, the color looks so natural and pretty... I can't see using anything else. Good luck!
Has anyone here used henna on their hair?
I just ordered some and after years of staying away from the chemical stuff, I am excited to be "messing" with my hair again 
I have almost-black hair, so I got a red color and the jet black in case I need to cover up the red color, haha.
I have read about using different liquids for different effects....chamomile tea for high lights and coffee for darker tones, et cetera. Does anyone have any experience doing that?
I am also REALLY interested to see how it affects the texture of my hair. My hair is super thick and healthy (-looking, at least), but my sister in law has thin, wiry hair and I am wondering if henna might help her bolster it up a bit!
Any tips?
