Screen legend Richard Burton enters the story as Father Lamont who is sent to America to investigate the death of Father Merrin [Merrin met his demise at the end of the first film]. Burton gives the role of Father Lamont all he's got and provides the only memorable moments in this film. Without him, it would be bleak indeed.
The problem with Exorcist 2 is that it gets bogged down in pseudo-science and religious mumbo-jumbo. It's got too many things happening at once and they don't necessarily tie together very well. We have Louise Fletcher [Flowers in the Attic, Firestarter] who is trapped in the role of a sympathetic psychologist. She gets my vote as "most likely to loose her license by the end of the film." As someone who works at a hospital I have to ask "Who wrote this crap"? Then there's the incomparable James Earl Jones [Star Wars, The Lion King] who finds himself wearing a locust hat and sitting in a grass hut. Seriously? This is Luke Skywalker's father for crying out loud. Show him the respect he deserves!
To add insult to injury, the sets in Africa look like they were constructed for Disneyworld and the annoying soundtrack from the usually brilliant Ennio Morricone makes me want to drive an awl through my ear drums by the end of the film. [They say confession is good for the soul so I'm glad I got all that off of my chest.]
I don't think Exorcist 2 is as bad as the critics say it is, but it's not stellar filmmaking either. If you liked the first one, skip over Exorcist 2 and watch Exorcist 3 which is the worthy sequel to the original.
For more info check out the film's entry in IMDB.