Dreamfall is the sequel to The Longest Journey, and I wouldn't recommend trying it without having played the first instalment: returning characters and concepts are only explained cursorily, if at all. Furthermore, if you thought The Lord of the Rings: The Return of the King had a lot of endings, or if you hate cliffhangers, don't play this game, because it sure has a lot of endings, and all of them are cliffhangers. And the third part(s?) are nowhere in sight and unlikely to emerge in the near future.
Oh, and if you want to play this with the mouse? Forget it. Get a joypad (I used the Xbox360 one).
With that out of the way, what did I think? Dreamfall is an extremely pretty game with a very well-written storyline and a stupid tacked-on combat system that does little but suck (but also does not mar my enjoyment of the game all that much).
A quick digression about action sequences: I hate action sequences in adventure games. I'm an adventure purist, and had to force myself to finish Broken Sword 3: Sokoban and Broken Sword 4: Sneakasmatron. Space Quest 3 with its endless action sequences near the end, is still on my list of worst adventure games. I'm really bad at hand-eye coördination, and these sequences are therefore annoying stumbling blocks to me -- I had to play the fight at the end of SQ3 more than 50 times (if not a hundred, memory mercifully fails me here) before I got it. Therefore, if you include action sequences in your game, prepare to get a bucketful of hate from me.
I found the action sequences in Dreamfall to be unnecessarily tacked on, but even I could finish most of them on the first try. I have to label the combat system entirely broken. As long as you rush an enemy and keep hitting, they mostly just stand around looking stupid until they die, and you're never in any danger. When there are two or more enemies to fight, they will happily stand around awaiting their turn to be slaughtered. Why was this included at all?
The sneaking sequences make more sense, but here the other problem with Dreamfall comes in: the camera isn't good enough to support this fully. Fortunately, the other characters are all completely deaf, and the sequences are, again, pretty easy.
Lastly, there are "hacking" sequences, which are time-limited minigames. Also not very hard, also tacked-on and unnecessary.
The strength of the story absolves Dreamfall of all these problems. Even in the last fourth or so, where the game falls into the good old "let's make the player run all across the world to fulfil fetch quests" rut for a while, the story kept me going. The game is very cinematic and switches between three playable characters and in-engine cutscenes to create a "movie" experience quite unlike any other game before or after it. I can't really say much more without spoiling the experience, and in this case I don't want to -- go play and enjoy TLJ and Dreamfall.