I gave up on it years ago. Aside from not being very well written, it just wasn't medieval enough for me and the overly politically correct casting choices jarred. You could argue that black actors and glass in the castle windows shouldn't matter as its fantasy, but it just didn't work for me.
On the subject of black actors in primetime family drama, Robin Hood was much the same. But oddly, the cast of Primeval, set in the present day, is pretty much white. Odd.
Anyhow, Merlin has apparently improved - will they get to finish the Arthurian arc with the final battle against Mordred ? It would be a shame if they didn't.
I think I'll dig out my copy of the brilliant Excalibur and give it a rewatch...