I've listened to 2 podscasts - the PVP + PA podcast over at WotC, and the Expedition to the Ancient Academy podcast. The PVP/PA/WW podcasts were all very enjoyable, and were probably the best introduction to 4e outside of actually playing myself. As you'd expect the sound quality was consistently pretty good, and everyone playing was clearly having a good time.
Expedition to the Ancient Academy lacked a little something in sound quality, but was a nice old school listen. Having the map that the adventure was happening in helped too.
Unfortunately that's the only experience I've got with actual play podcasts. What are your favorite podcasts? Recommend me something!