I remember this being discussed last year some time, I'm not really a big fan of the idea since it just makes life easy for managers in terms of selecting a bench. Having 7 subs lets you select (depending on availability, obviously), say:
Keeper
Centreback
Striker
Winger
Holding midfielder
Fullback or utility defender
Creative midfielder / different sort of forward
With only 5 subs your options are a bit more limited and you have to be a bit more cautious. e.g. say you have 4 quality midfielders/forwards who can't get in the team, with only 5 subs you can't really risk putting them all on the bench plus a keeper, because then you'd have no defenders.
The only positive I can see really is that this brings things in line with european/international matches.