I've got a Playmaker MC, an Aerial defender DC/DR, a Defensive wall DC, a Free-kick specialst MC and a Penalty-kick specialist ST. I'll just share my experiences with them:

Playmaker - used to lead the Top passers table, but he isn't that effective lately (he did score plenty of goals though).

Aerial defender - one of my initial players and has played in every single match so far, so I can't make a decent comparison. I do often see him heading clearences from him in comments though.

Defensive wall - bought him a week or two ago, haven't received a goal from a free kick since.

Free-kick specialist - one of my latests signings, but I've seen enough to confirm what everybody says - 75% of the times I get a free kick (excluding the ones on the sides, where only crosses are made) it's a goal. I have to say though - first free-kick against a stronger team is never a goal.

Penalty-kick specialist - I've only received a penalty once and once I have played a penalty shoot-out in a friendly match. Didn't have this player on both occasions.