The Mekong River trip from Huay Xai to Luang Prabang is a popular (some say essential) part of a Laos/Southeast Asia trip. However the journey takes 2 days, the slow boats are uncomfortable, and the fast boats are dangerous. There are other options.
The slow boats are basic and uncomfortable. The take 2 days, stopping the first night at Pak Bang, which is not a good place. The second day they continue to Luang Prabang. You can see pics of the slow boats here, here and here. The slow boat trip costs US$20 to Luang Prabang.
The fast boats (speedboats) are considered dangerous, and are often stopped from running by the authorities when the river level is low. The fast boats cost US$28 and take 6 hours. You can see pictures of the fast boats here, here, and here.
More luxurious boats
There is also a more luxurious boat called the Nagi of Mekong. This boat used to go from Huay Xai to Luang Prabang in a single day. However, apparently the slow boat mafia have pressured the operators to stop doing this. Therefore this boat now does an overnight stop in Pak Bang. It costs US$120 per person, including twin shared accommodation in Pak Bang, food & drinks on board and transfers to and from the piers are both end of the trip. It leaves Huay Xai on Sunday and Wednesdays.
For a luxurious option try Asian Oasis who run a trip from US$250 per person. The 2 day, 1 night trip is impressive.
Phoudoi Travel apparently sell tickets for a comfortable 34m boat with chairs, tables and food & drink on board. Tickets are US$60 per person, with the trip taking one day to reach Lunag Prabang, leaving every Monday and Friday. I suspect this could be the Nagi of Mekong boat above.
If you have any information or tips about these boats trips please leave a comment below. Thanks.
Some people say that the Huay Xai to Luang Prabang section of the Mekong is not the best. It is possible to more scenic trips in Laos, particularly by taking the 3 hours bus from Luang Prabang north to Nong Khiaw and taking the 7 hour boat trip back. People have suggested this is a great Mekong River experience.
Also consider entering Laos, from Thailand, across the friendship bridge at Nong Khai. You can reach here from Chang Mai by bus (12 hours) and take a shorter and probably nicer boat trip when in Laos.