a1) Once to one person, when I was new to the forum.
a2) I don't think I did? There's a comment system with the reputation system?
a3) That I had read enough from this person to think their opinion would be valuable in general. I didn't expect anything in particular.
c) My home forum is 4chan so I dislike the system. Pro is probably that the most cursory user can quickly determine whose opinion has the highest probability of being truthful/helpful. Con is dick measuring contests and other things to do with cliques and drama.
I haven't been to the big G4E DLC thread and only pop into random threads I find interesting so perhaps my understanding is limited, but so far Affect3D forums in general don't seem to be plagued by the problems of this system design. Then again it doesn't seem to be particularly benefiting from it either. The most helpful/insightful posters are usually the ones with high post count or a username that isn't in plain black font, so reputation is actually the least useful of available indicators.
That there are no problems or benefits either way I think is due to low population though. I would probably not frequent a forum of high population using a reputation system (e.g. Reddit).