Simply the best!
Once I got addicted to Once Upon a Time, I decided I wanted to hear what other people thought about it too as they will often point out things you might have missed. I subscribed to four different podcasts including this one and listened to an episode from each show. ONCE podcast was the only OUAT podcast that I listened to more than once. In fact, after listening to all of their discussions on the current season, I intend to re-watch OUAT and listen to their podcast from the beginning as I do so.
Their podcast is informative, insightful and just the right amount of humour. Plus Daniel J Lewis has a voice that is perfect for radio. It kind of reminds me of the way someone like Ira Glass(from This American Life) speaks. Where his voice alone pulls you in and makes you want to hear what he has to say from the second he opens his mouth. Not that the others(Jeremy and Erin, I believe) don't sound good as well. Their voices work well with his and all three seem to have great chemistry.
In closing, if you can only listen to one podcast about Once Upon a Time, you should be listening to ONCE podcast.
UPDATE: I've fallen behind on OUAT but after each episode I find myself wanting to hear what they were thinking when the ep came out. It's a constant battle in my head: listen to the podcast or watch the next episode. At least 1/2 the time, the former wins. Also, I really love it when Erin compares the show to Buffy. She often apologises for bringing up the similarities but it's my opinion that she has nothing to apologise for as the comparisons work well for the two shows.