I know I am late to the party, and there have been pages of responses to the question. If someone has covered the material I am about to offer, I apologize in advance, I have not read every post in the thread.
From personal experience, I will from now on carry a rifle that has an adjustable cheek piece when I anticipate shooting long range. There are two very important issues that made me reach this decision.
1) 20 MOA Picatiny scope base rail, coupled with a 56 MM objective lens on the scope I prefer. This combination makes even the highest comb on a convention hunting rifle too low to maintain a good cheek weld when in a less than perfect shooting position.
2) Physical stress and fatigue cost me a very nice 300 plus bull Elk, and I don't want to miss another shot because I couldn't maintain a comfortable and relaxed position.
We were filming my elk hunt in Arizona last fall. I was shooting a McMillan EOL Outdoorsman in .338 Lapua. The Elk was 840 yards across a canyon and was a great trophy. I had a slight incline, took a position over my backpack and got ready for the shot. As I settled in Bob said "You can't shoot yet, we don't have enough light." The sun was directly behind the mountain where the elk stood and the closer it got to peaking or the ridge the more critical my eye relief became. Because I was not able to get a good cheek weld and still see through the scope because of the position I was in, I started to get fatigued. I was up on my elbows, my neck was tiring from holding my big noggin up and by the time the light was adequate I was trembling in my upper body. As a result I was frustrated that with just the slightest movement the field of view went completely gold and I wasn't having much luck controlling my body, I pulled the shot and sent the 300 gr Berger right under the elks nose. Because the sun rose over the ridge within 20 of the shot, I never saw the elk through my scope again and though he stayed put, I never got another shot.
I honestly believe that had I had and adjustable cheek piece and been able to rest my cheek as well as my head on the rifle, I would not have tired so quickly and would not have had the issue of roaming in and out of the field of view as a result. I am pretty confident I would have made that shot had I had an adjustable cheek piece on the rifles stock .