*sigh*

Problems I've had with Aggron (WARNING: PROBABLY BECAUSE I PLAYED EMERALD) :

-It's a bitch to evolve (Lairon sucks).
-It has two 4x weaknesses (Fighting and Ground) and is weak against Water.
-There's no point in modifying it (I equipped Lairon and Aron before Aggron with Macho Brace and it's awful).
-There are WAY better solutions. Aggron may be able to use a ton of different moves, but a weaker Special Attack makes most of those moves pointless, not to mention it doesn't get a STAB bonus for the most important Attack techniques, like Earthquake and Brick Break.
-It has a bad Special Defense. I can defeat most Aggrons in one surf attack with Swampert. If I can't it's because of level difference.
-Once you finally get an Aggron, it's just not worth it since you're already close to the end of the game and you can only use it for the one of the many tedious* gym battles (you'll probably get it by the time you reach the Mossdeep* and Sootopolis gyms). Sure, you can use it in the Elite Four and Victory Road (or Battle Frontier), but by that time I could have developed a far more superior team.
-Aggron is a steel type, so it's also slower than most of its weaknesses.

Any questions? These are just my observations and experiences. I just never found Aggron to be that useful or important enough to stray from developing a better team and Pokemon that can have that STAB bonus for important moves like Brick Break and Earthquake.