I have been using Bazaar-NG for about 6 months or so, during which I have been trying to keep up with their mailing list discussions. Before Bazaar-NG I used Subversion and Monotone. I like the idea of "Distributed VCS", and so far Bazaar-NG seemed like the best one at that for me. That is, Bazaar-NG is still a work in progress, and they do some great stuff there, with great ideas. I'm a fan of Martin Pool and Aaron Bentley, two of the drivers of Bazaar-NG. Martin Pool is a great VCS designer with great patience, it seems. He is the leader and maintains the main repository. Aaron Bentley has many great ideas that he adds in his plugins project (BzrTools). One of the cool things that I used in BzrTools was the push/pull or something like that. But then there is this PQM thing which should be a patch manager, which I generally don't really care about, but they need it. Thus, Bazaar-NG moves sometimes more slowly towards the designs than I wish, and some of its design ideas like PQM don't really relate with me.
The first time I read about Mercury it didn't seem quite ready as well. But now I feel like giving Mercury a try. If it proves to make client/server with "distributed" well enough, while supporting Windows with confidency, I might switch to it for another 6 months, giving time to Bazaar-NG to mature beyond the initial design goals. Also, maybe this relation of Mercury with Git may be attracting many people to Mercury, because their mailing list seems to be quite active as of late.