Silverberg hits his stride around the mid-'60s and themes eventually emerge. "Flies" is about a man granted a measure of omnipotence and the not-so-nice results. In "Passengers" an alien occupation takes the form of intermittent takeovers of human bodies wherein the aliens usually party hard with their borrowed vessels (kind of the ultimate drunken blackout). Soon there's the repeated trope of human attainment of immortality in "Sailing to Byzantium", "Born with the Dead", "Capricorn Games", etc. These stories concern people out at the limits of human power, or sometimes powerlessness.
Where does this stuff fit in in the universe of sci-fi works? Well, to use a very basic taxonomy: Telepathy and time travel? Check. Aliens? Quite a few, but around the margins and rarely as characters. Spaceships? As needed. But robots? Computers? Not so much. It's more about yearning and transcendence than about technology or society or especially plausible futures. For all the themes of human perfectability, the point of view is usually that of a confused, overwhelmed, and/or manipulated protagonist, and in the end most here is about the feelings of smallness and inadequacy, or the limitedness of human existence. However, there is definitely a lot of feeling-- you may get a little misty when the wistful melancholy of loss and uncertainty really hits. Lovers in these stories have very romantic barriers to overcome. There's vivid language and often very vivid, dazzling settings (the "ancient" cities of "Nightwings", "Saling to Byzantium", etc.)-- you can tell Silverberg worked hard! (Especially since he mentions it a lot in the introductions.)
So it's a decent soft-headed mystery-of-man's-place-in-the-universe sort of science fiction. Not as deep as it might seem, but often very unique, heartfelt, and well-written. If you can humor the occasional over-seriousness, quite entertaining. And actually there are a few good lighter yarns. I had never read any Silverberg before, but this volume feels pretty comprehensive-- a good place to start and to end too, probably. I'd guess it would be most suitable for harder-core crate-digging sci-fi fans. I suppose I am one, but I found it worthwhile.