I go to the park frequently (and have on occasion sprung for FL+), and agree that if you aren't a local, and especially if it's your first ever trip, to get it for at LEAST one day. Yes there are ways to avoid lines but you have to have an active plan and go on rides in a certain order, and frankly if you are on vacation, you shouldn't be worrying about all that and FL+ allows you to wander around and ride and rest and eat as you please. I go all the time so I don't mind having a plan since I can't afford it every visit, but I can't stress enough you should budget fast lane for at least one day. All 3 if you can! I wouldn't recommend buying it ahead of time, as weather and other conditions can change crowds dramatically during certain parts of the year.
TAKE ADVANTAGE OF EARLY ENTRY! DO NOT SKIP THIS EVEN IF YOU ARE GETTING FL+.