I love henna. I've used it three times now - once as a gloss, and two full applications. Pure henna - the real stuff - only comes in red. That's the color of the dye the plant releases. My natural hair color is medium and dark brown (with some gold mixed in). After the second henna, my hair is a deep deep ruby red/brown ( I call it cherry-glazed chocolate) when outdoors. I get a lot of compliments on it, even though I think it's really subtle. Indoors you can still see it, but not as much. It won't show up that much on black hair.
Henna also made my hair thicker - so just be prepared. But my twists now look great, where they were kind of limp and wimpy before. My loose hair still looks the same, but my twist ponytail looks thicker.
Another thing I noticed is shinier, softer hair. The softness kicked in even more after the second henna. Henna is like a very long-lasting conditioner - the shine, softness, and strength last for a long time. I noticed that it took longer for my hair to absorb water after the first henna, but that went away after the second treatment.
Just keep in mind that if you have used chemical color on your hair, you have to be VERY careful about the henna you buy. Compound henna (sometimes falsely sold as pure henna) contains metallic compounds that will interact with the chemical color, damaging your hair and turning it black or green. So be careful what you buy.
I have the results of my henna gloss and 1st henna in my Fotki.