Well, I've actually had experiences with a toddler with both the old fast pass system and the new one, and I definitely prefer the new one. Mainly for not having to be the fast pass runner for the family. You can also move times around via your iphone app, which we actually did when it started to rain and we decided to eat lunch early. It took all of 2 mins to pull up my fast pass on my phone and change my time to about 3 hours later..which worked out great because it stopped raining...so anyway....
My only real gripe with the fast pass system is that it limited your choices to 3, which I hear is fixed now.