Hi Wendy. My wife and I did an overnight cruise with Handspan Tours a couple of years ago and they were wonderful. They are also one of the more eco-friendly tour companies in the Halong Bay area, parts of which have become quite polluted in recent years, due largely to a lack of tourism regulation.
Regardless of who you go with, I recommend choosing a company that will take you to neighboring Bai Tu Long Bay. It is less crowded and less polluted than Halong Bay, and the natural scenery (particularly the limestone karst formations) is just as stunning. (I believe that Indochina Junk is another company that is licensed to take tourists to less crowded areas of Bai Tu Long Bay, but I have never traveled with them myself, so I don't know what their quality is like.)