Is mirror lock up worth it? Depends what kind of photography you're doing. It's up to you, but I've found it to be significant even with shorter lenses when using a tripod. See:
http://www.worldphotographyforum.com...ad.php?t=1037&
I agree with Stephen about use when hand holding though. A few times I've forgotten to turn off MLU and taken a hand held shot. Framing the image is "interesting".