Aggron...it's still a great pokemon in my eyes. I personally believe people are utilizing the "wrong" set. As the stats clearly show that majority of the players are using the RP set...I am greatly disappointed. I still believe his CB set should be the one that's would allow Aggron to shine the most. Sadly, it's not happening :(
Aggron is a funny poke...
Just like Porygon 2, i think it works better on OU than UU. In UU we have many Aqua Jet, Mach Punch and Vacuum Wave users that can revenge RP Aggron. There, it has to deal with Intimidate Hitmontop, Quagsire.
In OU, there's only Swampert and Hippowdon as counters(Swampert can't swtich in many times like Quagsire). And priority there is less threatening than it is in UU.(there's 2 Mach Punch users: Breloom and Infernape. Both rarely uses said move).
In OU, there's Sandstorm to let Aggron switch in more easily on special attacks. And in OU there's more baits for Aggron to switch in(Scarf Latias, Scarf Rotom using Shadow Ball, Scizor, Skarmory, Salamence trapped on Outrage, Blissey...) and attack with Head Smash, which hurts pretty much everything that does not resist, and some that do resist(i believe Machamp is 2HKOed with Stealth Rock for example).
With Rock Polish, you need to be sure that there's no poke that can KO Aggron after that. Since Aggron resists lols at one of the most used priority moves(Extremespeed) and is neutral to another one(Bullet Punch), it has little to no problems with revenge kills. Beware of Scarfed pokes with a SE move though.
Choice Band lets Aggron hit whatever switching in hard, potencially killing it. Its job is to eliminate pokes through sheer force.
In UU... let me say, it is harder to use it there. But it is still a good poke there, walling Choiced pokes like no one.