They are not rare, lots were released. No practical difference to A570 and just like A570 it is practically worthless.
Not as sexy as Viper A530! Neither of these cards were made in volume, both are like rocking horse poo and demand overly large prices for what they are.
I've said it 100's of times, the best upgrade for A500 is A1200.
Not sure thats correct. These were the prototype that was given out only at shows and the like. When officially released it was rebadged. There is an interesting article on one of the CDTV fan sites about the rarity of the unit, and from the mail I get I would suggest few are in existence. Happy to be proven wrong. Always pleased when there are more Amigas around than you think.