I've seen this show up to the end of Season 4, so I don't really know what happens in S5 yet.
I too thought it was basically Smallville made by the BBC. On that note, it was entertaining if you knew to expect the same level of writing as Smallville.
As a plus, Merlin didn't "permanently" lose his powers every 3rd episode, only to regain then right at the nick of time, thru some contrived loophole in the story. In this aspect, it was better.