I missed 90% of AG, came back in the midst of DP (right around when Ash got his third badge in America; roughly would've been when the Summer School arc started in Japan, I believe), and I picked back up just fine. It's easy enough to get back into, and you can just look at all the overly important stuff by going to YouTube. (And believe me, there's not as much of it as people would like to let on.)