Correct me if I'm wrong, but as I understand it, shackles DO cause the spring pack to sag faster. Granted, putting on the shackles and driving back and forth around town won't cause any change.

But like you said, they add wheel travel and cause the spring pack to move in a longer arc. IN offroad situations, combined with the removal of the rear sway bar, this extended arc travel will cause the spring pack to fatigue and flatten much quicker than it would w/o the extended shackles.

I'm debating between the two right now as well, and will probably go w/ the shackles b/c they're cheaper (damn gas prices), and will just start saving up for the SLR spring pack when the time comes.