The reviews I've read have all noted that the picture quality is an improvement over the DVDs -- but not a massive improvement, especially when it comes to the visual effects.
The extras sound interesting, but they're certainly not worth the current price. I'll wait for them to go on massive sale in a few years.
I think I'm in the same boat. Especially since seasons 3 & 4 were filmed in HD.
That's inaccurate. If you mean shot with HD digital cameras, then it's only Season 4.
All other seasons were shot with 35mm film (which can have far more detail than HD) and scanned into 1080p, so they're also HD.