"The Dark Shadows Companion" states:"In November 1969, after nine months of some of 'Dark Shadows' most intricate, intelligent storylines, an end came to the adventures in 1897.
As for my own personal opinion, I liked the Leviathan storyline. The one I couldn't stand was the whole Adam plot. How I wished somebody would've tugged on that one, little stitch that was hanging lose and unravelled the whole thing, meaning both that annoying creature and the story.
The writers had a golden opportunity for some real soap opera with the return of Paul Stoddard. If they had allowed for that to be separate from the Leviathan plot, it may have been better off.