Well, even though the puzzles are fair and logical they are sometimes rather hard if you are not used to thinking around several corners. I recommend having a good hint-system ready if you tire of puzzles easily. Because, really, the story, characters and atmosphere are so worth it. My recommendation: Unversal Hints System which you can find here: http://www.uhs-hints.com
/ Unlike a walkthrough it only gives you the hints you need. And sometimes, I slight nudge in the right direction is all you need. All three GK-games are available for UHS. Other than that, the versions you get here are pretty much completable without any additional tinkering.
This answer, on an otherwisely long dead topic, is aimed at new players:
GK1 is the only game ever where I tried UHS and got disappointed. That's because hints are not sorted by what problem are you trying to solve, but instead by location. So you sometimes get instructions like "Now use the screwdriver on the machine" and you start thinking " Oh great, now where was I supposed to get this screwdriver???" which was what you actually needed to know all along.
It gets worse. Some items may be picked up at one day but only used in another. If you happen to skip them the first day - like I did with the scratchbook - you end up looking for a hint that is buried a few days earlier.
I found a plain FAQ from gamefaqs immensely better for this game. I can't say it applies for GK2 or 3 tho. I'm only 90% through GK1 and haven't touched the others so far.