On further viewing...
Discovery HD has the second best HD picture (next to the amazing BBC HD) and also has the best programming at the minute. It also features amazing 'Discovery HD Short Feature' films, that so far have ranged from deep sea diving with an HD camera on a sunken ship engulfed by coral (wow!) to a man wrestling an anaconda. On the negative side of things, it features the biggest/most obtrusive DOG of any HD channel.
National Geographic is probably the worst picture wise - very grainy and a few artifacts. It really doesn't do HD justice. Also the shows tend to be 20 minutes of staring at a river with no voiceover and very little to enjoy watching.
Sky One HD probably has the best DOG - very small and only in the top left hand corner of the screen. It's pictures are comparable to the mid range stuff on Discovery HD but not quite reaching the peaks of the best of Discovery or BBC. Bear in mind I've only seen Enterprise to judge from though.
Sky Sports HD - I've only seen content shot in HD, scaled to SD and then upscaled again. No live HD broadcasts for it until Thursday afaik.
Artsworld is alright but hasn't shown anything I've been dying to watch so far, their shows seem to be less geared towards showcasing what HD has to offer and focus more on the show's subject matter without jaw dropping visuals.
I'm really enjoying the experience so far, and while it isn't worth the £300 yet, I know it will be when the World Cup hits, as the BBC HD trial footage of World Cup 2002 is absolutely jaw dropping. The sense of scale and intensity in the pictures beggars belief.
I'll be watching Starsky & Hutch in HD and Shrek 2 later on so will report back with more impressions.
Anyone else lucky enough to have it, bookmark Discovery HD for 9pm on Saturday - a show about the making of Star Wars and how real world technology is following in it's footsteps. The shots of Episode 1 in HD were great looking.