Why would you need to replace iOS' built-in podcast management? There's only one reason, really: if I want to get new podcasts on my device, I have to physically sync it (or go through a ridiculous process of finding that podcast in the iTunes store and downloading it or syncing it). That's annoying.
So, what does Instacaster having going for it?
- It's got the basic functionality you need (stream, download, play, pause, double-speed)
- It's got OPML support (as of version 1.2) to let you easily (not quickly, but easily) add your podcast subscriptions
- It let's you pseudo-automatically download new podcasts right on your phone, without having to sync it
That last bullet is pretty much the big one. You can refresh your list of podcasts (on 3G or wifi) and pull down any new episodes, reasonably quickly. So, if you're away from your computer and need something new to listen to, there's always something available, and it's easier than the iTunes phone interface.
What's not so hot?
- The interface isn't the most straightforward thing ever. Expect a good bit of "oh, that's what that button does" and "wait, why isn't this thing cached on my phone" until you figure it out. It'll only take you a few tries.
- It's hindered by a fundamental limitation: the iOS APIs won't let you add songs into the device's music library. That means that I have to listen to these tracks inside of the application. That's fine, but it makes it much harder to listen to stuff through my car's iPhone interface. I have to keep track of where I was at for when I get to work and switch to using my computer and headphones (whereas podcasts in the iPod player simply sync their location and you're good to go.)
The app is $2, so regardless of the limitations, it's worth it to me to have the convenience, but if you're on the fence, wait until you see iOS 5. It wouldn't shock me to see the podcast interface overhauled, rendering the app obsolete.