I'm largely ambivalent when it comes to "twist" endings (e.g.: endings where everything, at the last minute, turns out to be vastly different than what it seemed). When they're done well, they can make for a great literary experience. Fight Club, for example. On the other hand, they can sometimes feel a little telegraphed, which I feel may be happening in this short story.
I won't get too much into the twist of this story, as knowing the twist ahead of time kind of ruins the story. However, I will say that all the signs seem to point to the twist early on. Don't get me wrong, I enjoyed this story, as I tend to enjoy all of Ellison's work. More specifically, I loved the setting he creates in this specific story: a future world where psychic agents are trained from birth to protect the presence from dangerous creatures from the future. Yes, it does sound lame when I say it, but Ellison makes it seem pretty cool.
What I didn't love is that the ending twists all that around in a way that seems so, well, obvious to me. Maybe it's because, since this story was written, others have copied the idea to such an extent that it just doesn't seem fresh? Or maybe it's just a weak point in the story. I'll leave that to you to decide for yourself.