Cadfael is an amazing show that I remember well from when I was a kid. This is the only Mystery program that my parents would let me stay up for when I was younger. Recently I decided to look them up again, and started with the books, then moved onto the show.
The transitions are almost flawless. Of course there are things that you miss, and subtle nuances that can't be translated. Still, the writers do an admirable job of condensing the 200 or so pages in each book into a coherent, thorough story.
Derek Jacobi is, of course, amazing. I cannot imagine anyone else playing Brother Cadfael. Sean Pertwee was my favorite Beringar, or at least the one that captured the Hugh from the books the best. I also though the relationship Pertwee and Jacobi's characters was the closest to portraying the deep, brotherly love and trust between Cadfael and Beringar.
At the center of every Cadfael is a damned fine mystery. I can't say that the twists and turns are Holmesian, but the end is surprising enough that you don't get bored. Wrapped up in the plot is usually an endearing love story. Not saccharine or distracting, but sweet enough to keep you involved, and a good way to show that even in murder mysteries featuring monks, there is still a place for romance.