It's hit and miss. I played 5v5 elim for about 8 hours straight and it was fun as hell. At the same time I played public control for about 6 hours today farming exotics and crucible rep. Think the private matches are quite good for clans or people with bigger friend groups. Either way I doubt they will see a huge amount of use until a few weeks after rise of iron. Everyone seems to be gearing up for the dlc, which you can't really do through private matches.