I'm certainly having some mixed results at the moment. Some courses that I'd previously subscribed to are showing phantom episodes and no amount of deleting and redownloading makes them go away. I have two courses from the Open University that are each a single iBook, but both courses insist on opening the same book. I can open each individually from iBooks, but not from within the iTunes U app. When using the Catalog I can sometimes download material to view on demand but not subscribe to the course.
In each case I suspect the answer is to track down the contact details for the course provider and get them to look into the problem. For example the contact for the OU is itunesu-editor (at) open.ac.uk. Tracking down the correct contact can be a challenge... I eventually found this one from the main OU page in iTunes, then down the right-hand side was a section called "OU Links" and under that one called "Contact the Editor". I'm just about to send in some feedback so time will tell...
Thanks for your replies.
On a whim, I revisited the iTunes U tonight. No change - still can't download anything. Whenever I try, I get the same "Unable to Download Podcast" error message. Pity. So much interesting material. For now, however, iTunes U is nothing more than a good-looking icon on my home screen.
I have canvassed these forums and tried every remedy that's been suggested - all to no avail.