I'm playing in a region with some friends and have built the Arcology.
The density is currently level 8, and there are 1,093,306 people in there now.

Benefits for my city (so the game says)

 - Shoppers: 13,175
 - Workers: 664,331
 - Students: 415,800

Still, my schools are kinda empty, businesses close because they don't have workers,
and housing demand is still high for $, $$, and $$$.

So to be honest, I haven't seen any real benefits yet... but I could be overlooking something.