Not sure about Artsworld, content overlap with some of the stuff on HD1 I would have thought but if it's bundled into a single package no problem but I can see myself paying for it if it's standalone.
Discovery Theatre HD has some stunnign stuff on, watched Alien Planet last weekend and it was one of the most intersting and enjoyable programs/documentaries I've seen in a long time and of course looked stunning.
The Star Wars specials they had a while back were also mind bogglingly good and some of the clips of the recent trilogy made you want to weep in gratitude to Lucas (well putting aside the script and some wooden acting of course).