I'm at like 70% completion after 3 days. 100% Story / 90% Sidemission and I've just begun doing the challenges. Dude - after like four sessions of doing sidemissions, going into the Riddler Challenges is like heaven. I almost forgot how much I loved these challenge rooms in Arkham Asylum; and now there's like 500% more of them. The core gameplay mechanics are so damn good in their pure distilled 'gamey' form. When one spends most time backtracking and figuring out Riddler's hidden secrets, it's easy to forget. Hidden Secrets are obsessive fun, but them Challenges are pure joy.
I'll definitely go for getting all the medals in the challenges, before I go back and finish up the Riddler collectibles. Might even get to the 100% mark before BF3 comes around on the 27th/28th. Didnt' think I would go for it at first, due to the overwhelming mass of completionist content, but somehow I'm almost done and it was a fun journey. Rediscovering my love for the Challenges makes 100% completion very likely. Go for gold Batman!