Like a lot of people here, I learned to program on a 99/4A before moving to a PC. The 99/4A was actually great, but looking back very primitive. TI-Basic -> Extended Basic -> Assembly. When I got to the PC, I missed hardware sprites and a real sound chip (a sound card for a PC cost $300 at the time - about $100 more than my TI cost my Dad!).
It always amazed me that the 99/4A had such a vibrant community (that is still creating hardware and software around the machine) and so many outlandish ways to expand the machine.