The reason this service is so valuable is the EASY factor. You've always been able to podcast for free, but normally you would have to maintain several different accounts and it's quite time-consuming. With PortableSermons you simply upload your audio file and they do the rest. All the formatting, getting your sermon on iTunes, everything. It's so fast and easy, you can literally upload your sermons and have them ready to download before your congregation gets home from church.
Anyway, check them out - they're a good company. One of the owners/creators is also going to be doing our church's web design. His web company is VaultMultimedia.