Massive, comprehensive, heavy. Those who have fallen asleep in bed while reading The Essential Ellison have been known to suffocate under the tome's tremendous weight.
If your personal library has only one square foot to spare, however, this is without a doubt the best single source of all things Ellison. It is the only place, for example, where you'll find Ellison's earliest published work ("The Sword of Parmagon," a serial, written when the author was but 15 years old, that ran in the Cleveland News), his first professional sale ("Glowworm," from the February 1956 issue of Infinity magazine) and one of his many unproduced, unpublished screenplays (this one for the "Our Man Flint" television series).
The editors' attempt to arrange the stories and essays thematically doesn't always hang together, but they offer a wealth of bibliographic data and more than a little analysis, most of which seems right on. The book really is essential.
The second edition adds more stories and another 15 years of a remarkable career.