I first came to Cat Ba in 1998 on an overnight tour by boat from Halong City. At the time, the little bay of Cat Ba was choked with beat-up old fishing junks, and the quiet little oceanside strip had but a few budget hotels. A sturdy Russian Minsk motorbike was the main mode of transport. Oh, but the times they are a-changin'.
The population of little Cat Ba Island is now over 30,000, the bay has been cleared of junks (all fishing vessels must park a few hundred meters offshore), and the main seaside avenue has been widened. There are a number of midrange high-rise hotels, and even a luxury resort on the Cat Co III beach just north of town. Direct high-speed boat service runs from Hanoi via Haiphong, or, even better, try the direct bus that goes from the town center, rides on a ferry, and delivers you to the center of Haiphong or on to Hanoi. The town is very busy with Hanoi weekenders, especially in the summer, and thus best avoided then (the hotel prices are often nearly double on summer weekends). Most travelers zip through on 1-night package tours, but it's best to arrange your own transport to this area. Tours to Cat Ba limit you to dinner with the group and a strict schedule. I recommend booking a tour with a group for transport only or making Cat Ba your last stop on a budget tour of Halong Bay, and then arranging your own hotel and making your own schedule for your time in Cat Ba. The Cat Ba National Park has some good trails to explore, and the interior of the island and the rocky eastern coast are great places to get off the map (contact any hotel tour desk for an all-day tour of the island). You might want to check out the April 1 festival that commemorates Ho Chi Minh's visit to the island: It's quite a grand fete, with dragon-boat races and a big fashion show.