Come talk to us if you're interested. We are stabilizing things and hope to open up evaluations more in a week or two.
Though you are right, the architecture we tried earlier, when we were doing a proof of concept, would not have helped with compression. Back then we were trying to play nice with mongodb's data format and keep its legacy indexes around. It turned out that for many reasons (concurrency, recovery, and compression, and a few others), it was a lot better to replace all of the storage code, so that's what some of us have been doing the past couple months.