I remember these. C64 developers actually seemed keen on it at 1st. Potentially it gave the c64 a chance to have some more elaborate software due to the cartidge format. If it wasnt for this Shadow Of The Beast wouldnt have been ported to the c64. Sure, the c64 itself has a cartridge slot, but for whatever reason some developers seemed to think that a dedicated console would make c64 cartridge software more feasible.
Games like SOTB, Turbo Charge and Last Ninja 3 were released on cartridge because of the C64GS, so while its not something Id have bought in the day, there are now some nice collectables available because of it.
