i cannot answer you on the rarity since there's just no way to know.. you can see from the colour charts in the ref library that it's a seasonal colour (spring 07). the evergreen colours are black and white.
I have anthracite (that's it in my avatar picture) and it is a good color for every day, if that's what you're asking. It's dark enough not to show a lot of wear or dirt, and the color is very versatile.
It was only out for one season, as others have said, but wasn't limited edition like magenta (where they only made 500 or so.) Depending on the style and hardware you want, it will be hard to find new at this point. I made probably 15-20 calls to stores back in early July to find my RH anthracity, and there were only a few GHs left then too. You may need to check eBay and reputable consignment stores to track one down.
I have an anthra city and it is an absolutely gorgeous color. When I was looking for one a couple of months ago, the only one I could find was in Australia and I would have had to pay way over retail for it. I passed on it, and a couple of days later one popped up on Ebay that was brand new so I grabbed it. Keep looking.....You never know! Good luck!