First of all, if you are a really big fan of the Arthurian legend, stay away from this one. Even I, with my superficial knowledge regarding those ancient stories, could tell that this show takes quite a few 'creative liberties'. This said, it is really a rather good show. The sets and costumes are nice (though way out of time for the historic era), the storyline is interesting with a good mixture of humor and suspense salted with some adequate special effects. After just three shows in the series it is hard to judge how much depth the main characters will be able to develop, but they are all so.....likable (My favorite being the young Arthur). There are probably better ways to spend your Saturday evenings, but if you're in front of the tube anyway, you can certainly pick worse.
EDIT:
I just finished watching the ninth episode. It is hard to believe, but what appeared to be another shallow if decently made series, turned into a respectable drama show. The humor from the first three shows is still there, but as the story unfolds so do the main characters, gaining depth and subtleties and opening interesting plot opportunities.
Die hard fans of the legends will still not like it, as much of the underlying mystery gets a complete work-over (and the presence of a dragon sure doesn't help there), but as a stand-alone show, it certainly got me hooked :)
If you haven't been able to see it, because you're not from the UK and have no access to internet sources: This is definitely worth waiting for!
EDIT2 (a few years later):
Five seasons and a couple of years later I can still say that it's been a good show. Even though it eventually fed off the chemistry between the main protagonists and was weakened by the sub-par acting of what was supposed to be the main villain and antagonist, it holds its own as a work of entertainment. Good quality family entertainment!