Julian Frisch asks: "How do these committees affect your (personal) daily work in the Commission? Do have frequent contact to member states' experts from these committees?"
It really depends in which DG you are working in. In my case, I would say that we do have to deal with a committee quite often. There are regular meetings with the Member States experts, but they are definitely not our direct bosses. With time, you start to preempt their comments... It is all about national interests, so pretty obvious to guess.
Regarding the quality of the experts in the committees, it varies a lot. Once again, it relates to national interests. They make sure they have knowledgeable people in areas dear to their agenda. You can also imagine that some countries are more inclined to protect their own interests, for example those with large industrial powers, let say.