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.

By the Numbers
Just click on the thumbnail for any key in the timing sheet and press any number key to set that frame to hold for 1-9 frames. If you want to set it to be longer than 9 frames press the control key and FlipBook will let you enter a 2 digit number for up to a 99 frame hold.

Drag n Drop
You can also select any thumbnail (or group of thumbnails) in FlipBook and drag it up or down to move it forward or backward in the timeline.

Slide 'Em All
Or you can select any thumbnail and press the Alt Key to slide it and all the keys below it up or down. This even works with several levels selected at the same time.

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.