My experience with exchanges is very limited: I wrote a yuletide treat once, and then signed up for Purimgifts one year, didn't give any instructions, defaulted, and never commented on the fic I received (which was not a fandom I was super into, but had listed so that I could get a match, and used characters I didn't really know -- not a bad fic, but not one I had much to say about!).

The treat I wrote (which is my only fanfic on AO3) was well received and made me think that I should write more fanfic -- but I'm not sure if exchanges are the way to get into doing that.