See my blog: http://www.ich-war-mal-kurz-weg.de
We did the Halong Bay with "Cat Ba Ventures", a local tour operator lead by Mr. Tung, who starts his tours from Cat Ba Town on the island of Cat Ba, about 50km south of Haiphong.
Cat Ba, in contrast to Haiphong (at least what I heard about it), is much less crowded and secluded. The harbor from which we started contained only like 5 boats that day. The trip there was kind of worth it, because of the Vietnamese countryside and the bus-ride across the island.
We started in the early evening, sailing into the sunset, buying some fresh seafood and relaxing on the upper deck, drinking some beer and having fun. The idea of doing it with a group we know was probably the best we had.
The next day we went kayaking, swimming, doing a short hike through some caves and ended up eating even more delicious seafood.
The crew was very nice: three men were on the boat with us. One as english-speaking leader, the one second as cook 'n captain and the third one as helping hand (I suppose).
So: I really do recommend Mr Tung and his company "Cat Ba Ventures" as a tour operator!