Depends on all manner of things:-
1. What model car ? - eg is it a basic Corsa or a Porsche?
2. Whether you are the sort of person who services your car every 6 months anyway?
3. How much the plan is and (exactly) what it covers and , more importantly, doesn't cover.
4. Is it a main franchised dealer ?- do you currently get your car serviced there? If so, are you happy with their performance?
5. If you haven't used the garage for servicing before do you know someone who has and is satisfied?
How long is a piece of string???