It depends on your goals and preferences.
If you prefer at-home workouts, but need a little guidance and motivation, a workout mirror can be an incredible tool. They're also a great tool for people that want to be able to do a lot of different types of workouts in a smaller space.
They're usually not super useful for people that prefer gym workouts, want to lift a lot of weight, or prefer in-person group classes.
Another thing to consider: Fitness mirrors are pretty pricey, so if you're not absolutely positive that you'd actually use it, consider getting a mirror from a company that offers a trial period *and* free returns.