FlipBook is a great playblast tool because you can easily edit the timing while the scene continues to play in a loop. Just import your key frames into FlipBook, set it to loop and click on Play. FlipBook will continue to play and show you your changes in real-time while you simply move the key frames back and forth in the timeline in of three different ways.
Slide 'Em All
MayaLink is a script that automatically sends your key frames to the PC version of FlipBook and then sends the timing changes and reference drawings back to Maya.