I did my first programming on one of these, translating text games (bitmapped graphics were possible with the 16K RAM pack, but I didn't have one of those for a long time) from a book of them written for the TRS-80. The combination of having to translate between dialects of BASIC and having to "translate" game features to match the ZX81's reduced capabilities was a great practical exercise in inventiveness and perseverance. I particularly remember a Star Trek combat game that ended up taking up so much of the onboard RAM (one whole kilobyte!) that it started eating into the screen buffer, reducing the number of lines the computer could display. I think I ended up with one or two lines of text telling me what was going on, and one line of input for my next command.

Today, I work at Amazon as a senior software engineer. I believe having access to this affordable[1] computer as a teenager helped make that career path possible.

[1] The TRS-80 and other contemporaries of the ZX81 were very not-affordable to my family at the time.