You're looking at it wrong. To use a reach weapon properly, you have to be defensive. Because if you just go up to 10 ft from the enemy and hack at him, yeah, next turn he'll just 5 ft step in. The enemy needs to come to you, so you can get that AoO.
The other thing is, reach weapons alone actually ARE pretty worthless, except maybe for countering big things with long natural reach so you can go offensive on them w/o eating an AoO yourself. To make reach weapons worthwhile, you need feats. Combat Reflexes, obviously. But also Improved/Greater Trip and/or Stand Still. Something to use that AoO on to not simply hurt them, but to stop their movement in its tracks. You then become a sort of martial battlefield controller.
I must admit, with the nerfs to the combat maneuver and Stand Still feats, the insane difficulty of winning a combat maneuver check in PF's new system, and the introduction of the Lunge feat (+5 ft reach, but only on your turn, so it helps the longsword guy hit you safely, but does nothing to expand your area of control) and ESPECIALLY the Step Up feat (any warrior can get it at level 1, and if they do, once they close to adjacent with you, you can NEVER safely disengage to get 5 ft away to use your own weapon on them), I think the combat style is significantly less useful in PF. To the point where I question its validity at all. I have not made a martial controller w/ reach weapon in PF yet, and have no plans to, even though I make them often in 3E.