There are lots of reasons to love single-player Gottlieb EMs.
- parts availability (this is important when restoring/maintaining EMs!)
- artwork (generally non-licensed themes, and the art has stood the test of time better than others)
- ball to ball strategy and buildup (unlike a multi-player, where the playfield resets on each ball, the single-player Gottliebs let you achieve targets over the course of 3 or 5 balls). Even the Bally single-player EMs reset the playfield on each ball! Ugh.
- build quality and robustness
- general availability (you can still find 'em)
- market value (they hold up reasonably well, especially the better titles)
- peer pressure
Yeah, there are some titles that aren't so hot, but overall, it's hard to go wrong with a single-player Gottlieb EM in decent condition.
[Disclaimer: I'm heavily biased, but that's from being at this for awhile, and sampling the EM spectrum.]