I have!

They feel a lot different. The Brooks is more firm, and the An-Atomica is more like a hammock. I was very comfortable on the an-atomica until it stretched too much and the edges started to cut into me. I had it replaced once under warranty, but the next one did the same thing- just not as fast. My experience seems to be unique- I think a lot of people never have that issue.
My Brooks was also very comfortable, but I need a cutout for riding in the drops. If I had to recommend one, I'd say get a Brooks from Wallbike- you can't go wrong with a 6 month return policy!