After having paged through the Rules Compendium as well, many of my reservations about where Heroes of the Fallen Lands left me have been alleviated. It seems that they basically split the PHB in half for the Essentials book. The Rules Compendium copies and pastes a large amount of the material present in HotFL, but also adds in everything missing from it from the PHB. I think, overall, it's again nothing new, but it's a much better book in terms of actual substance. I think that even if you own a copy of the PHB, it may be worth getting a copy of the Rules Compendium as a "table book," because it's much better organized and contains all gameplay-related things, on top of being smaller and lighter than the PHB. It does not, however, lie flat, so you need to prop it open or leave a bookmark if you want to keep a page in mind.
Overall, it's again nothing new, but as a tableside companion (or if your copy of the PHB has fallen apart, like they are so good at doing), it may be worth getting this as a replacement. It's like the Quick Start Rules, except has all the relevant gameplay rules as well. A handy reference guide, nothing more.