Yeah, Head Smash is awesome on Aggron, but I don't see a bright future for this Pokemon in OU when there are better options available, such as the 'mons Bad Ass mentioned. Aggron's nasty weaknesses to many commonly used types (particularly Fighting / Ground attacks, which are everywhere at the moment) also means that it runs into serious problems with several prominent threats. These include Roobushin, Garchomp, Terakion, Birijion, among others.
Defensive threats that resist Rock, like Hippowdon, Bronzong and Natorei (these two must watch out for Fire Punch), also trouble Aggron. Aggron's nasty Water weakness and weak Special Defense give it problems versus rain teams and the powerful special attackers they tend to run. If anything, Aggron is out shadowed by Aron in OU, who's level 1 Endeavor set is surprisingly effective if you can keep entry hazards off the field.
QC REJECTED 2/3