We just came back from a trip (1/3-7). Went to MK day 1 and didn't purchase the $15 LP though my wife was able to buy them for one ride for a lesser amount. But the crowds were up and we ended up spending an additional $75/day for the LP since we had someone who had not been since she was a kid and her child who was there for the first time. It did save some time and frustration and guaranteed we got into attractions like the Mickey train ride at HS.

We waited to buy them after midnight and probably could have gone without them on Friday since the crowds were down at MK and Epcot. Instead of using LP, we opted to hit RotR, Avatar, and Smuggler's Run right before park close on subsequent nights, basically giving ourselves extended hours since they allowed you to ride when you were in the queue. Plus I like walking through the park at night when the crowds are gone.

If you are visiting the parks during the busy times (seems like all the time now) or have some "must see" popular attractions, you may want to budget for the LP.