I love henna too. I usually add a couple of tablespoons of ACV to it to help it hold onto the greys. It's like a recipe, EVOO, ACV, crushed rosemary, hot water. I have medium brown hair and left it on for an hour last time, it's now like a burgundy, but not really bright or dark, it's still subtle enough to pass for natural color. And the way it looks in the sunlight is amazing! I'll upload a picture of my henna'd hair on my profile if you wanna see.
It usually lasts about 4 weeks or so before I start to see my greys again, and the great thing is, that you don't really have a defined root line like with chemical dyes. At least not in my experience.
A good henna will also be a very fine powder without sticks and other large plant pieces in it. And as others said, beware the "black henna". You want 100% pure henna, nothing added. Be sure that when you rinse, you rinse a really long time. A really long time. The olive oil does help to make rinsing easier, but it's still gonna take a lot to get that stuff out.
I put plastic wrap on my head and wear either a black stocking cap over that or an old towel because the heat is important in getting the color to release from the plant and attach to your hair.
Anyway, go for it. It's super good for your hair, and you don't have to worry about roots or yucky chemicals. And it's much less expensive than a salon visit. What's not to like?