I have actually read The Decline and Fall, but I'm still completely absorbed by Colin's extended precis of it. His style is concise and perceptive, and the easy, informal delivery is never stuffy and often genuinely funny, in a Gibbon-esque sort of way. Think Mike Duncan. The underlying subject-matter is of course vast and sprawling, but he cuts through that, seemingly effortlessly, though I suspect a lot of prep must go into it.
The only problem is that the of course the Decline and Fall, though very long, is finite, so at some point this podcast must come to an end. But at time of writing Justinian is still alive, so we have something like a thousand years to go. Great!