We have used:
# Arcana
Critter ID, magic items etc. Lots of use from this one.
# Architecture and engineering
Whenever I cast shape stone or wall of stone to make something structural, my DM hits me with this one. Which I don't have. Damn.
# Dungeoneering
Yup. Again, it mostly seems to be used as K: Underdark, but hey.
# Geography
Never. Survival does weather prediction a few days in advance AND keeps you from getting lost, maps exist and travel magic (which doesn't care about terrain) exists too. This one seems wholly useless.
# History
Useless. No, really. Either the history in question is irrelevant, but nice flavour text, or it's just going to be given to you by the dm. And if you're really stuck, legend lore isn't that expensive.
# Local
As long as this is left generic, it's great. If a DM decides that k:local only applies to the place of your birth (or a specific area, or whatever), then it rapidly becomes useless.
# Nature
Animals, fey, giants, vermin and monstrous humanoids. Wooohoooo!
# Nobility and royalty
Eh. If a guy's the king, you know he's the king. And if he chooses to get all offended at your manners, you can probably give him the smackdown. Take it if you want to hang around in court, and that's about it. In general, diplomacy does every this one does and more.
# Religion
Undead. They're so complex, you NEED this one. If nothing else, just so you can identify that a critter is undead and can therefore be turned.
# The planes
Outsiders. See undead. And the fun you can have when you know about the planar traits... Everyone's sick? Let's just jaunt off to the positive energy plane for a bit then!