When I first started gaming, this was the only way I played. The friend who introduced me to D&D ran solo games for me, and since I didn't know anyone else who played at the time that was how I started running games. I had been DMing for about two years before I started running games for more than one player, and since then I've pretty much always been friends with enough gamers to run for/play with groups.
I ran a lengthy solo campaign for my best friend when 2e first came out; we played regularly (several times a month) for about two years, and semi-regularly for a little while after that. When the campaign began, he was a party of one -- a fighter. As time progressed, I introduced his PC's brother, who was a ranger, and two other fighters. Other NPCs rotated into and out of the party; off the top of my head, I can remember two thieves, a gnomish illusionist and another fighter. Despite the party composition, we spent at least as much time roleplaying as running combats.
We were pretty flexible about who ran the NPCs -- he almost always ran his brother, while I usually ran the other two fighters and any rotating party members, but sometimes he ran everyone.
In terms of game balance and structuring the challenges my friend (and his party of NPCs) faced, I didn't use much in the way of traps or magic -- our game was very political and human-centric, and the longest-running portion of the campaign was centered around a war in the Moonsea region of FR. We were both learning about gaming as we went along, so I would introduce stuff as I found out about it, and he would change his playing style as new elements of the game became more interesting to him. For the most part, I just made sure that he and his party were tough enough to take on what I threw at them -- and in the habit of recognizing when they
weren't tough enough, or a spellcaster was going to take them apart, etc.
The campaign began in Taladas (in the Dragonlance world), moved briefly to Ravenloft, then to Spelljammer, and then settled into the Forgotten Realms. We had a blast.
