Like the Gardner books I've been reading and enjoying, John Varley's The Ophiuchi Hotline is part of what I call the Old Universe subgenre -- when humans hit space, there were already ancient, inscrutable, and powerful aliens there. This is a subgenre I've always enjoyed, largely because it allows for high idea-density, deep mysteries, and cosmic stakes.
Also, I suspect that there's a bit of nostalgia to my fondness -- when I think about the SF that I read and loved as a teenager, stuff like Niven's Known Space, Clarke's Space Odysseys, and Brin's Uplift (all of which are Old Universe books) are right up there at the top. In a way, even Douglas Adams qualifies.
Anyway, the point here is that I like this kind of book, and consequently liked this book; it had all the requisite ideas, mysteries, and cool worldbuilding. But I think it suffers in immediate comparison to Gardner's books: Varley is covering much the same ground, but does it with less wit and a less-well-developed plot.
Unfortunately, this book was written before the trend toward what some call "bloat", and what I call "giving the damn plot time to develop", so it goes along perfectly fine and builds up an interesting plot... then wraps it up quickly and unsatisfactorily inside of 20 pages. An extra 50 pages could really have helped this book out.
As it stands, though, it's still an enjoyable and fast-paced read with no major flaws and plenty of things going for it; if you like this kind of book, you'll like this book. I'm not quite pencilling in Varley's other stuff on my "buy immediately" list, but I may do so yet.