I listen to a lot of podcasts - A lot of stuff from twit, cnet, smodcast and npr. I sync my phone about once every two weeks or so which for a lot of my daily podcasts is not good. For twit there's a decent mediafly app that is specifically for twit but it seems buggy - when you go back to listening to a podcast after stopping for a while it goes back in time and will skip back anywhere from 30 or so seconds to about 20 mins and you then have to scrub to find out where you were. There is also a standalone mediafly app that is supposed to be for alternate podcast locations but it doesn't seem to do a good job handling the RSS and misses quite a bit plus it has some of the same bugs as the twit version. Also as with any background audio api (aside from the iTunes app) they never properly are able to display the right info on the lock screen - some times it shows the title of the app, sometimes it shows the last thing you were playing in itunes (even though it doesn't play that and plays what you want it to), sometimes it's blank - and it never shows the artwork.
Right now I've resorted to subscribing via iTunes on mac, then on my iPhone go to the podcast, get more episodes and them download from there. It works but it sucks if I'm away from wifi when I want to do that as there is still the 20 meg limit. I'd settle for a single download limit going away and a button to download all new episodes being wifi only
What I would like ideally is to press one button that will download all of the latest versions of my podcasts to the itunes app - this isnt likely to happen.
Failing that I'd like to have an app that lets me do the same thing, download the latest episodes with a single button, delete older episodes automatically, play nicely in the background, and have a good browser/management interface.