The Longest Journey popped up several time in the recent forum thread on "most disappointing game", so my main advice would be not to buy into the hype. Expect a well told story, nothing more, nothing less.
If you're into fantasy fiction (and to a lesser degree sci-fi), enjoy a well crafted game world and don't care too much about pacing or puzzle quality, you will probably enjoy it quite a bit.
It's certainly an ambitious game, and it truly feels like a labor of love - a quality I find absent in most current adventures with Telltale being a possible exception. But that very same love may also have left a few darlings too many unkilled, especially in terms of dialog but even whole side quests that imho could easily have been sacrificed for the sake of a tighter narrative. Personally I find that wears out its welcome around the midpoint and on my second playthrough I gave up way before that.
Comparing it to MI2 or GK1 is really apples and oranges. In terms of scope, world exploration and quirky characters it does bear some resemblance to MI2, but the story takes itself far more seriously (not to say that there aren't jokes, but rarely laugh-out-loud funny), and it's more of an epic quest than a wild adventure. I guess the dialog heavy narrative and more adult themes could be compared to GK1, but it wouldn't fall out in TLJ's favor.
I'm not trying to dissuade you from buying it, because you might as well make up your own mind about this modern-day classic. For better and for worse it's an important piece of adventure gaming history. But don't expect miracles, it's just a game
