Voadam said:
How do you use knowledge skills in your game?
How do you actually implement this skill in your games?
Is the skill a bonus that aids the player's gaps in knowledge on the subjects?
Is the existence of the skill a limit for characters that don't invest in it?
Do you have a problem with a player playing a knowledgeable character with no knowledge skill points (such as a tiefling fighter who knows the politics of hell and the DR type vulnerabilities among the hierarchy of devils)?
I generally eyeball a character's concept and history, in-game experiences and general investment of skill points/mechanical end number to see if I impart to the player information I think his character would know that he doesn't. The mechanics at the end are the least important part for me. I often don't even roll and just go with my gut on whether the information seems appropriate for the character or not. Determining DCs and then rolling just adds in randomness after I have to make a primary judgment on appropriateness of the information anyway.
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I also find monster info is a tough call on what is appropriate "useful information".
IMC the Knowledge skills are essentially an in-character option to get "hints" during the game. They are usually very useful skills, because they are used very often and because they can help the characters both figuring out the plot and preparing themselves for certain encounters.
There are typically 2 uses of these skills: either you roll generically about a subject (the higher you roll, the more info you get), or you roll against a specific question to see if you know the answer.
I enforce what I believe was the originally intended way that these skills should work: as a check to see if the knowledge of (or answer to) something is already in the character's mind. As such, I do not allow to take 10 or take 20, and I allow retries on the same topic only after a significant amount of time (usually, but not always, after the ranks in that Knowledge have increased). I understand why some DMs like the "recall/memory" idea (with which take 10 make sense), but I do not use it, just as I do not require to recall knowledge of spells, feats etc.
I allow players to define their own new Knowledge skills. I do not mind the existence of overlapping knowledges. There are some unavoidable overlaps anyway (e.g. if the subject is a Holy War of the past, both KN(Religion) and KN(History) can help), so for me even if someone has KN(Creatures) and another has KN(Dragons), that's fine for me (although it makes little sense for the same PC to have both). The narrower the skill, the easier to know something: to know the habits of a certain dragon type might have DC 20 in KN(Dragons) and DC 30 in KN(Creatures).
Overall anyway I'm not very fond of using Knowledge to reveal strengths/weaknesses of monsters in combat, so while I allow it of course, I won't bother too much if someone knows something from the book... anyway both the skill and the book may be wrong
One last nitpick: I disagree that Architecture and Engineering would be a Knowledge. For practical questions like "how much weight does this bridge hold?", I think Profession is a much more appropriate skill. For theoretical questions, KN is ok but in case of Architecture it is almost always useless.
