I was playing with a pre-production 79AVi since august, and now use a "regular" 79Avi since 1 month. The 79AVi is an improved 59AVi in all aspects, particularly the audio side. On the video side, 480i over HDMI is more "direct" and gives a more sharper picture, with less "ringing". The chroma problems of the 59AVi are now solved at the de-interlacer level, but not on the MPEG docoder (really disappointing...).
But make no mistake, paired with a good scaler and good display, the 79AVi is better then the 59AVi in the same set-up, PQ wise.
The price of the 79AVi could be alot higher, but with the upcoming Pioneer Elite Blu-Ray player coming really soon, and for the price they will sell it, they didn't had any choices... They are just putting the price of the 79AVi in line for the upcoming pricing of the Elite Blu-Ray.
I can't say when and how much, but I can say it's sooner then you think, and lower then you think...
So the price of the 79AVi vs 59AVi has nothing to do with what's inside. The 79AVi is heavier, bigger, and build quality is alot better. But Blu-Ray is comnig soon, so they put a price to go with that...