I’ve used it when planning the scope of projects and it’s been a helpful way of framing discussions with the various stakeholders. It generates some very worthwhile debates.
It really applies (IMO) to content of task lists, rather than priority or urgency. If an item is a Must, there is no option but to do it. It’s a good way of weeding out unnecessary work, if you’re able to be ruthless about it. But it’s not always obvious.
Shoulds are things that you should feel obliged to work at including in your activities. They’re important and ought to take precedence over Coulds and Wants. The trouble is, everything not a Must ends up in Shoulds, because there’s always a good reason for doing anything.
In a major project, these debates get resolved among stakeholders and against budget, time and resource. I think it’s a lot harder when you’re trying to manage a personal activity plan - especially when things can change from day to day (hour to hour, even).
I wouldn’t use MoSCoW in any formal way for personal task planning because of that. I might use it as a quick way of triaging today’s tasks, and I think it would be helpful there