Curses

This game is a puzzle galore. It is long. It is tough. It is great.
Starting from your mansion's attic, you simply have to find a Map of Paris, for your soon-to-be holiday trip. Though what this game does, is show you how a simple task can become incredibly arduous. You'll discover family memorabilia, curses, and travel time (and not only that). *Only* to find that blasted map. Nevertheless, don't let this banal task deceive you: Curses is full of atmosphere, and the stories you'll discover around your mansion - and around your ancestors - will totally capture you.

Again, this game is long. Both because it is big (very big, almost huge), than because the puzzles are so tough that you'll spend ages wondering how to solve some of the most difficult ones. But if you take notes (and you'd better - and you'll also want to draw an accurate map), you'll find that all the puzzles are quite logical, and this is extremely good for a puzzle game. The only drawback is that some of the logical deductions/connections you'll have to do are so hard that they're almost impossible, and maybe they might've been implemented better (but this doesn't mean they're badly implemented).
Al lot of the stuff you encounter is not considered (you might well find a table in a room description, and get a "you can't see such thing" message when examining it). But, for once, this is no drawback, because it allows you to concentrate on the important stuff.

On the bad side, sometimes Curses can be really frustrating. It is easy to get stuck (tough puzzles, remember?), and also to reach an unwinnable condition, because a lot of what you do is irreversible, and you might not be prepared. Though, if you pay attention and save often, you will catch the wrong actions soon enough.

Overall, if you are a puzzle lover, you HAVE to play this game. This will be a real challenge, and if you can complete it without any walkthrough, go out and buy yourself a prize: you're a genius (sadly, I was not, and had to recur to some help in a few of the most difficult situations).
If you don't like puzzles instead... well: go away ;-)

One last note, about a thing which is always given as expected, but which I'd like to point, for such a complex IF: this game must've taken many months of development, then more months of debugging, and IT'S FREE!!! A bow to Graham Nelson, and to all the makers of huge IFs out there.