Since about 2006 I started keeping my running log online. RunningAhead was my go-to logbook from the beginning because it was simple to use and free. Then a couple of years ago I started using DailyMile and really enjoyed the social (dare I say Facebook like) atmosphere. Now this year I started using Strava. I also use SportTracks desktop software, which has a bunch of open source plugins that let me track Training Load (TRIMP, TSB, etc -- more on this later). SportTracks also auto-uploads to DailyMile thanks to a plugin by Fellrnr. If I can just figure out how to get SportTracks to upload to Strava and RunningAhead it would really cut my data entry time down to a reasonable amount.
I may have one too many logs, but they each have something different to offer. One of these days I might consolidate into one log, but with GPS and automatic uploading it really isn't that difficult to manage.
The one thing I like the most about the running logs is the reassurance they give me that the training I am doing is effective. They also keep me accountable (to myself) and helps me stay focused on my long and short term running goals. Focus is super important, especially when the training plan I am following is 24 weeks long!