For those without available wired broadband options, the price point is a no-brainer.
In rural areas, your only option is often cellular (3G/4G) "hotspots" and the monthly cost will be outrageous unless you can convince everyone in your house to stay off of Netflix, etc.
If you're in a rural area and you want Internet access for streaming video or your kids who are in school or just don't want to have to deal with or worry about data caps (with outrageous overage fees!), this is quite likely your best ption.
The $100 monthly cost isn't unreasonable. You're going to be paying probably $60-70 for spotty cellular service with a data cap and/or throttling anyways, so might as well pay the little bit extra and not have to worry about it.
The one-time $500 setup/equipment fee will be harder to justify. I expect Starlink will offer an option to spread that out over a year or two which would make it much easier for some households.