So, I am running a 10th level wizard and have 30 concentration currently. Every level I put the max I can into it. I don't have any concentration items. However I am making my first hat from the Cove. I put the wizardry as the one slot and was thinking about the skill slot. I was going to go with Diplomacy as that is a weak skill given my charisma but really useful when I mess up aggro. So adding +15 would really help my diplomacy, +15 to concentration would not be as big of a total increase.
So, I read the wiki and I see how concentration is used but I am not sure if making concentration a really great skill or have two generally high skills is better. Honestly, other than Jump I am not sure how much of a skill is considered "good".