I first picked up the original printing of the first volume of thisanthology when I was a small child, around ten years old, and the first story in it ("A Martian Oddyssey") was so good that I put the book back down and didn't read the rest of it for another year because I was afraid none of the other stories in there could possibly be as good.
The second two volumes took me years to track down; II B I managed to find in a sale of discards from my school library; II A I didn't find at all until Amazon came along.
The conceit of this series is that the Science Fiction Writers of America picked the best short stories, novellas, and novels from before the Nebula Awards were commenced in 1965, and published them as a hall-of-fame anthology. Volume 1 collected the short stories and volume II (A and B) collected the novellas -- essentially, one stop volumes of all the "Nebula Emeritus" books, the sci-fi that professional SF writers of the sixties felt had most influenced and impacted them up to that point.
As such, this series is perfect for two groups of people: people who are completely ignorant of sci fi, and people who want to gain a better critical understanding of sci fi and its history as a genre. You can't find a better starting place, because these are the stories that the great modern SF writers started on, so by reading these, you'll understand more about what modern writers are doing, and you'll have the opportunity to experience the tropes first hand, from the stories that coined them, not in later knockoffs.
This volume (II A) I prefer slightly less than I and II B, if only because by the time I'd found it, I was older and had read some of the stories elsewhere and seen the tropes before, so it didn't have quite the same glow to it as the other two did, read in childhood; I also feel a couple of the stories in here aren't quite up to the same level as the rest. Still, there are some definite essentials -- "Universe" was the first generation-ship story, "Who Goes There?" is the source story for John Carpenter's film _The Thing_, The Marching Morons is an early version of the conceit in Idiocracy, etc.
Probably the best benefit of these volumes is that they'll give you a general familiarity with the big names of Golden Age SF, so that you'll know who you like and don't and whose works you want to find more of. If you're looking to expand your knowledge of Golden Age era SF, this series is an excellent place to start, and you'll probably find yourself tracking down most of the other works by most of these authors. I would, however, point you to Volume 1 first, especially if you're new to the genre.
This volume contains:
Call Me Joe by Poul Anderson
Who Goes There? by John W. Campbell Jr.
Nerves by Lester Del Rey
Universe by Robert A. Heinlein
The Marching Morons by C.M. Kornbluth
Vintage Season by Henry Kuttner and C. L. Moore
. . . And Then There Were None by Eric Frank Russell
The Ballad of Lost C'Mell by Cordwainer Smith
Baby is Three by Theodore Sturgeon
The Time Machine by H.G. Wells
With Folded Hands by Jack Williamson