I've got the one by Nick Kollerstrom. He prefaces the calendar with some detailed background into the different planting calendars available, which do conflict. There is also some reasonably scientific looking research which suggests that there may be some benefits to planting at particular times.
Personally, I use it for time management. So much to do, so little time - two or three days on a 10 day rotation for each of roots, flowers, leaf and fruit/seed, gives a plan as to what to when.
