I just use my tablet, with a word processing program. It is INVALUABLE, if you want to track when you watered and what with (plain or with nutrients). It's especially great if you include the days old/days in flower. I find I skip a day or so, if there's nothing new.
So far, a log had enabled me to enter notes about the strains I grow, how they did and any problems I run into. I expect, after an undetermined number of grows under my belt, I will learn what's best for me.