We were at WDW last week which was very busy due to President's week, Mardi Gras, Daytona 500, etc. We found the FPs most helpful at DHS and AK. At Mk, we didn't really use them but Splash Mtn was closed for refurb, so that might make a difference.
I decided not to use it at Epcot and we had no problems, so it was the right decision for us. We just got a regular FP for Test Track and rode everything else standby. At DHS it was worth it's weight in gold - Toy Story had a 130 minute wait!
It really does depend on what attractions you like - we like the roller coasters and ToT.