caliscurls,
I've used the chestnut bar two Saturdays in a row. Each time, I have lathered, and let the lather sit for ten minutes. I can definitely see color in my hair, but I still have a lot of gray/silver that hasn't been affected. Some of the grays have a more brownish tint to them, but of course, I always focus on the ones that I feel are still very obvious.
I think that the bar is a good way to experience henna without the mess, and the long waiting time. It probably isn't the most efficient way to cover gray hair - yet. From what I have read though, the more that the bars are used the more they will continue to deposit color, so I expect to see even more color this Saturday. If you've already been using henna, this might be an easy way to refresh the color/treatment.
I've been debating about using the auburn, but I have chickened out everytime. I wouldn't mind the grays being highlighted as a brighter red, but I don't want all of my hair to be reddish. I like my dark brown color. Interestingly enough, I actually see more color the day after I shampoo than on my shampoo day. Of course, part of that is probably because I usually bun my hair while it is damp. As the hair dries, the color always lightens.
I will probably get around to putting something in the "Hit/Miss" thread this month on the henna bars.