For its time, it was light-years ahead in capability for the money. But even though I already knew the 6502 assembly language it used, it was the power of what I could accomplish even in BASIC that amazed me.
When I bought my C64, I was just a few months out of college. Since I was working, I didn't have as much time to program for my C64. So that meant plenty of all-nighters - some nights I would program on my C64 literally from the time I got home from work until I had to go back to work. And even when at work, all I could think about was the project that work had interrupted. Some of my coworkers would catch me spacing out as I designed or debugged something in my head, and would tease me about it. Everyone knew that I was always working on home programming projects, and it became something of a running gag. "Kent's in the zone again, get a tissue and clean up the drool," ribbing like that.
In those early days, I wrote a lot of games in BASIC. The first game I ever wrote was the classic Battleship, complete with a computer opponent. I loved such logic games as a kid, and so wrote all kinds of them: Mastermind, Othello, and Backgammon to name a few, and all with computer opponents. I also played a lot of Avalon Hill Bookshelf games as a teen, so I wrote versions of a lot of those as well. One in particular was Stocks & Bonds.
My two roommates and I played a lot of Stocks & Bonds on my C64. For one session, I decided to play a prank on one of them, Dave. I secretly modified the program so that any stocks he bought would always go up, and my other roommate Alex and I would mostly avoid whatever Dave was buying. On the last turn of the game, all of Dave's stocks would crash. As the game progressed, Dave could not believe the luck he was having and was, of course, destroying us in the game. It was a struggle for Alex and I to not crack up as the last turn approached, but we managed. As soon as the C64 displayed the closing valuations, the look of confusion on Dave's face was priceless - right before Alex and I burst out laughing. Ah, computer pranks!