According to Cutler, ALA used in conjunction with DMSA is supposed to cure you up to 30% quicker: http://onibasu.com/archives/am/20727.html

My understanding is that DMSA chelates and pushes mercury out of the body through urine, while ALA pushes it out through stool. So if you have a poor bowel function, you might find it extremely difficult to chelate on ALA alone.