A few years back I did a one-night cruise on a junk (dragon pearl, perhaps?) with about six other cabins and my one regret was that I didn't stay longer. The problem is that the drive from Hanoi takes so long that you're not on board until after lunch time, then you cruise for a few hours, then stop over night and the next day you're heading back to Halong Bay again, disembarking before lunch. I don't think the scenery is so spectacular that it's worth a special trip there, unless you can find a nice boat and enjoy the cruise for the sake of the cruise.