One of the more intriguing parts of the Rules Cyclopedia, to me anyway, is the Other Character Abilities chapter. To get this out of the way: Weapon Mastery sucks. Hell, if I had remembered to add it to my house rules my current Nightwick Campaign would use the LotFP small/medium/large weapon system.
However, I really like the idea of General Skills. A few players in my old group often complained about the lack of character customization in OSD&D (I mainly ran S&W at the time but they also played LL). I think General Skills add a modicum of that without the cumbersome nature of 3.x's skill system. It would help to differentiate say a Northern born barbarian from a knight without the need for separate classes.
It also provides a method of healing that doesn't require someone in the party to be a Cleric. Now, I like Clerics and would never remove them from the Nightwick Campaign as they fit so well in that context. However, the idea of a Cleric as a designated healing class bothers the shit out of me. The healing skill is a nice way to mitigate some of that, but it's limited healing potential still keeps the spotlight on the Cleric.
I would introduce them to my game, but I'm not sure if I want to muddy the LotFP Common Tasks system I already use. How does one add an Ability roll based stealth skill to a game where any character has a 1 in 6 chance to sneak? I'm not sure yet.
I will say that I'm very tempted to adapt them into the Underworld setting. Since I'm thinking of using the S&W White Box as the rules for that setting, I don't have a dedicated way to handle opening doors, climbing, sneaking, and a myriad of other tasks. Also, since I don't have to accommodate Thief characters, introducing General Skills is that much easier.