I own 5700 105 and put a few hundred miles on a 6700 equipped Roubaix Expert. It should not shift like crap but 6700 is part of Shimano's first line groups with under bar tape mounted derailleur cables. Apparently they didn't adjust the pull ratio and so the added resistance on the cables degraded shift quality somewhat. The internal cable routing on the Roubaix Expert doesn't help either and adds a touch more resistance (my Roubaix has external cable routing and it shifts slightly better.) The new Ultegra 6800 should fix these issues but only you can decide if it's worth the money to upgrade.