After Longest Journey, I thought Dreamfall was a huge step down, in terms of gameplay. The puzzles were too simple, combat boring. But the story...DAMN! Dreamfall had an amazing story. Having played through Dreamfall Chapters, I think it's a very worthy sequel. Better puzzles, great music, and the story is just as good, with a number of great characters.
As for "should I play earlier games", I would definitely play the first Dreamfall first. This is a direct continuation, and I would imagine the story would be difficult to follow if you haven't played Dreamfall. Dreamfall is very easy, so you could play through it for the story without much effort. The Longest Journey isn't as important - you may want to treat that game as a prequel, to be played after Dreamfall Chapters. It has a different main character, and only a few characters come back for Dreamfall (though the ones that do are kind-of important). At the time of its launch Longest Journey was the most amazing point-and-click adventure ever, telling a great story, having high resolution graphics, and fixing a lot of the problems with the genre. But these days it's old-school, frustrating and antiquated.
That does not sound good. Maybe I should give this one a miss.
