I played them last year. I have...very mixed feelings.
I'm one of those people who prefers puzzle-adventure (Riven and Machinarium) and action-adventure (Ico and Prince of Persia) to full-on adventure games. I tend to just get so annoyed with the "waggle your cursor all over the screen until you find something to click on, then try combining it with everything in your inventory until it works" mechanics of pure adventure games.
The story of The Longest Journey was...honestly nothing special for me. It kind of dragged and never really pulled me in, and the story of "there's a parallel world where magic really exists" is kind of overused at this point (though they usually don't set the mundane world in the future in those stories, so TLJ gets points for that). However, I absolutely loved the characters and dialogue. April was fantastic, and minor characters like Captain Nebevay and Roper Klacks were hilarious. The characters, dialogue, and certain parts of the story bump it up into the low B category for me.
Dreamfall was like pulling teeth. The stealth and fighting were dreadful, Zoe's character model creeped me out every time I looked at her, her character itself was pretty dull compared to April (who, speaking of which, just made me sad in Dreamfall), the Ring-like parts with Faith flickering on the TV screen were embarrassing, and the segment set in Japan was just bizarre. The puzzles were less frustrating, but...I actually can't remember them at all a year later. However, I did like the story better than TLJ's story. It was much more interesting and had more truly emotional moments, but it still didn't entirely do it for me. And of course there's the fact that it didn't go anywhere because the rest of the story is waiting in the sequel which doesn't even have a release date, so for all we know the story could come crashing down completely.
As far as pure adventure games go, though, I've just picked up Syberia and Syberia II. I believe I'm about halfway done with Syberia and I'm really loving it. It might even end up in my top 10, depending on where it goes. I like that it's more tightly focused and that it's more about character development than an epic quest to save the world. And the puzzles make a little bit more sense too. Just a pet peeve, but I hate how arbitrary the puzzles are in most adventure games. At least in Syberia the game is about crazy people and their wacky mechanical inventions, so the puzzles make some sense.