I don't have much experience beyond the basic with Sequel (DataMapper is my drug of choice), but a simple test here shows that require 'sequel' makes my IRB memory consumption jump from 15.2 to 18.6 megabytes, while require 'dm-core' makes it jump from the same 15.2 to 26.6.

Of course it is by no means a scientific or fair comparison, but still makes me wonder.