Against all expectations I quite enjoyed that.
It’s very far from the horrible teen travesty I was expecting, and nowhere near as glib and “modern” as the BBC’s Robin Hood. Indeed it’s about as literal and sincere a take on an Epic Fantasy novel (by way of Harry Potter) as you’re likely to see on TV. The cast is good, and Merlin himself is a nice mixture of wit and self-deprecation. The atmosphere is well-served by some effective digital matte paintings and a rich John Williams-esque orchestral score. Even the dragon is well done, albeit exactly the same as the last few talking dragons I’ve seen1. I particularly like the fairy tale quality of Gwen-from-Torchwood’s plot.
Probably the worst-judged aspect is the Jocks-and-Nerds relationship between Arthur and Merlin, but it’s a long way from grating.
Whether the world actually needs “Arthur and Merlin: Before They Were Famous” is another matter, but the pre-Arthurian premise is almost incidental to the fact that this is a surprisingly decent bit of Epic Fantasy2.
1 In films I mean. Rumours that I’ve been seeing talking dragons are scurrilous and should not be listened to.
2 i.e. Entirely hackneyed and predictable, but in a good way.