I use it. My previous employer had its 401(k) program with Fidelity and the financial planner in their office showed me how to tinker with it to the best advantage. The tool has enough flexibility built in to enable a detailed simulation with varying times and settings of planned retirement age, future expenses, pensions and other benefits. Expenses can be divided into essential and discretionary. I quite like the display options: time horizon, graphical or tabular format, and desired level of detail.
Average, Below Average, and Significantly Below Average are three possible stock market return scenarios. The score represents the expected percentage of expenses at retirement according to the chosen scenario. Mustachians will probably chose the Significantly Below Average, which should give a nice margin of confidence. Like many calculators, the scenario it draws is more accurate for those closer to FIRE than those just starting out the journey.
My chief complaint is that a single account has to be designated to one goal only, which makes it hard to model accounts that support more than one goal, such as FIRE and kids college