The overnight ferry is very much worth doing, especially from Stockholm to Helsinki. The the first couple of hours you sail through the Stockholm archipelago, which is one of the highlights of travel in the Nordic region.
The Silja line tends to be a bit more expensive than the Viking Line, but tends to get a slightly better clientele (loud drunks can be a problem). The Silja line ships are nicer, with a long, airy, internal walking street with glass elevators going up the atrium wall.
The buffet can be a bit of a zoo, but if you view it as an anthropological excursion to see the effects of free alcohol (wine and beer are available on a help yourself taps) on Scandinavians rather than a refined dining experience, you might enjoy it. The are also some OK a la carte restaurant as well.
Last edited by Amexpat; Aug 3, 2012 at 6:27 am