I'm running a 3.5 Realms game, and it's probably the most used book in my game after the 3 core books.
It does have the feel of being a huge book of errata and it is kinda haphazard (some feats and prestige classes here, some Vile, Exalted and Psionics stuff in the back, spells and domains over here, some stuff about Faerunian planes here, although the plane stuff can be adapted to the proper Great Wheel cosmology with only minimal effort), and a little about the progress of the storyline/metaplot, but the contents are invaluable for a 3.5 Realms game. They took some of the more broken things from 3.0 Realms (like Incantatrix) and rebalanced it, although even the PGtF has errata now (one important one, Daylight Adaptation isn't a Regional Feat anymore, so it can be taken after 1st level.)
That and the art for the "Barrens of Doom and Despair" is a really great bit of artwork for the lower planes, one of the first pieces of planar art I've liked out of 3.x, I actually used it in my game as an example of what the lower planes looked like when my PC's passed through there.
So, if you're running 3.5 Realms, get it, otherwise, I'd say it's a low priority item (but can be mined for classes/feats/spells for other games).