So, I finally completed the JNCIA-ER today. I actually took and failed this exam on Friday by 2 measly points. Funny, I found the JNCIA-M to be a cake walk, but the ER was another story.
I used the Juniper Fast Track materials. Which include the OJRE student and lab guides. I found them to be barely sufficient enough to pass. I needed other resources to pass this exam:
JUNOS Enterprise Routing