He was a fine human being, and a far better author than most people gave him credit for. He created two undying archetypes for science fiction: the first being the Berserkers, robotic combat machines whose destructiveness outlived the war for which they were created. So powerful was this archetype that it was ripped off repeatedly, sometimes by people who had no idea with whom the concept originated.
The other archetype was that of the modern, rational vampire who tells his own story. The Dracula Tape, in which Dracula was given his own sardonic voice, was the first example of what turned out to be a hugely successful genre. Anne Rice and many others owe him a huge (and so far as I know unacknowledged) debt.
Fred's non-series work show a highly individual imagination at work: Octagon, Century of Progress, and Love Conquers All are not only very different works from the Berserker books, but are each so distinct that it's hard to believe they were all works from the same hand. And The Veils of Azlaroc is so freaking strange that it's clearly a candidate for the Weirdest SF Novel of All Time.
In person, Fred was soft-spoken and reticent, but had a sly, understated sense of humor that I wish was more apparent in his fiction. During one of our first meetings, at a Halloween party, I found myself staring at his teeth with great unease. (He had commissioned a dentist to make him a set of highly realistic vampire vangs.) When he encountered a young, enthusiastic Dracula fan who said that meeting him made this was the most important day of her life, Fred replied, "Fortunately you are young, and have many days ahead of you."
During his final illness, Fred woke one morning after having dreamed of chorizo eggs from a local restaurant. His family got him the eggs, which he enjoyed. The next morning, when asked what he'd like to eat, he replied, "I have had no prophetic dreams about breakfast this morning."
I'm going to miss him a lot.