This is the D&D 4E book that my group uses the most. Since we play online, having a PDF copy that I can quickly CTRL+F and search is very handy. The PDF is bookmarked, so even if I can't remember what term to search for, I can find the right section quickly and jump to it.
The rules are clear, well laid out, and take into account several errata that you won't find in the original Player's Handbook. If you play D&D 4E, even if you ignore the other Essentials books, I recommend picking the Rules Compendium up.
[5 of 5 Stars!]