I have never seen bad Fast Pass Lines. The longest lines I encountered were at Soarin' when it first opened. Even then, there were several CM's helping people to get their Fast Passes as quickly and efficiently as possible.
Regardless, I think the general concensus is that the Fast Pass lines are not bad but there may be that one ride exception.
And if it's really important get the Fast Pass early. At certain rides, (like Test Track @ Epcot & Everest @ MGM) I have seen them run out after the park had been open only a few hours.