The first video game that I played that I felt I really regretted purchasing, that I felt was truly bad was Gex: Enter the Gecko for the original Gameboy.
I had played the original for the N64, and I enjoyed it enough (I was young and naive). I bought the it used for the Gameboy several years later thinking that it might be fun, after all I enjoyed the console version, how bad could it be? I was mistaken, it was terrible.
I was put off buying used games for nearly ten years until I forced myself to buy the original Paper Mario for nostalgia's sake. Paper Mario on the other hand was an excellent purchase, made slightly redundant by the fact it would come out for the Wii's Virtual Console not but a few years later... where I would in fact, buy it again.