OK, I procrastinate, but finally got around to playing with the SNES game saver device. Since it came with no instructions, I am glad that a poster on another forum told me how it operates.
Basically, you push the right shoulder button in combination with the controller's 'select' button to save your game state, and you push the left shoulder button in combination with the controller's 'select' button to activate your last saved save state.
I tried it out with none other than Rocky and Bullwinkle. It seems that you are limited to a singular save state on the device at any given time since there is no GUI or similar at the game start. Basically, you start your game, hit left shoulder with select and the game goes to the last level you saved on. The main limitation is that it seems to not save in the middle of an given game level, but starts you at the beginning of the highest level previously achieved on that particular game. My guess is that you cannot have save states from more than one game on a device at any given time.
As for usefulness, this will be a huge help in completing previously too difficult games to complete. Just do not plan on using it for multiple saves or multiple games at the same time.
In conclusion, I wish there were similar devices for SMS, NES and Genesis. Can't really think of many games where save states would be useful from VCS/INTV/CV/5200 generation, or I would recommend it there as well.
As it applies to NES, think of games like Rygar, Castlevania, and Contra. Enough said.