Remember, computers fail. I dive with a computer, but if I am doing a dive where I know the specifics (depth, etc) like the Oriskany dive, I plan my dives prior to the day of the dive. Remember the old, "Plan your dive and dive your plan?" I knew exactly what mixture I was breating on each dive and how long we had at depth. My computer crapped out on my during the second dive on the "O" and because we did plan the dives so carefully, I knew exactly where I needed to be and when. On the other side of the coin, on normal dives, I do use my computer info during the dives, but I still fill my log book out with the table info just to see the difference in the two. Plus, being a DM(c) I need to stay up on it anyway.