The PMP formula guide has been very helpful to me. Getting an email every day is useful reminder to continue to study (in addition to the actual study content). Each note is easy to digest and gets you to study for an extra five to ten minutes a day - especially on an important exam topic that you might not be focusing on that day (or even at all).
I enjoy the regular exam tips and formula notes. These are great motivators and tool to keep focusing on getting that PMP certification and staying on track to study for the exam.