We booked the 3-day, 2-night cruise. The highlights were kayaking among the karst formations, the sunrise view out the window of our cabin, and walking through several interesting caves.
A couple of notes:
1) The R/T van ride from Hanoi is US$20 per person.
2) The 3-day and 2-day passengers are on the same boat. Day 1 and Day 3 for the 3-day passengers are the same as Day 1 and Day 2 for the 2-day passengers. For Day 2, the 3-day passengers are moved to a separate boat for the day, where you are taken kayaking (in a different spot than the first day) and caving and given the opportunity to hike to the peak of Ti Top, which has an excellent overlook of the bay. Unfortunately, it's also very crowded and chaotic there. Day 1 includes kayaking and a mercifully short visit to a pearl farm and Day 3 includes a visit to a quite spectacular cave and making spring rolls.
The boat itself seemed a little old but well maintained. The food is nothing to write home about, but not bad either. Don't come expecting a big cruise ship experience - we saw some people complaining about the lack of activities but remember, you're on a relatively small ship with very limited space and staff.