I'm using it since yesterday, so I can't comment that much on this, yet. I had three microB windows and FBReader running at night and the battery hadn't drained much the next day.
Did you have Tear running in the background? I suspect that Tear drains the battery as I had a problem the other day with a drained battery after having Tear running at night. I switched back to microB, did some tweaks and found out that microB ist faster anyway, except at Javascript, which I have disabled and only enable it selectively through the config script on sites that really need it.