I read a lot, and so don't have a lot of time to repeat books (too much new to read!) However, I've read this one at least a dozen times, and it is always just as good.
I force this book (and the others in the series...FINISH THE NEXT ONE, DAVID!) on all my friends at some point in their lives. Almost all of them have agreed with me about how good it is. I have four or five copies of these books around, just to be sure I never lose them when loaning them out.
One thing I will have to sheepishly admit is that I read this book long before I touched any Heinlein...and when I picked up my first Heinlein novel, Starship Troopers, I was floored to realize that Gerrold's style is very similar, and, well, Solomon Short has been around for a long time. Doesn't make it any less great, and it suddenly makes the dedication to Robert and Ginny Heinlein make a *lot* of sense!
The characters are complex, the hero isn't perfect, the science is very strong, the plot is interesting (it's a page-turner in several spots), and the overall sense of story from the four books together makes for some interesting speculations. I am sure David knows where it's all heading, but I hope he hurries up and lets the rest of us in on it...I waited years for the fourth book, and it was worth it. I'm sure that the wait for the fifth will be as worth it, but dang it, I'm getting impatient!