I've only kept a log for one build. It was more of a session log.....date, start and stop times, and what I did during the sessions. It was on Academy's 1/32 F-18D that I built for my bro during his first tour in Iraq (he was a WSO with VMFA(AW)-332 "Moonlighters" at the time). When he got back, we compared logs (mine is bigger!!!). Turned out, that at the same time (with time zones factored) as I was building the GBU-12s for the model, he had dropped a pair of em, getting 50+(confirmed) bad guys.
I also keep a log of finished builds...just scale, company, type....so I don't forget any for the "year in review" thread.