First off, I'd like to tell you a bit about what you WON'T find in this hefty volume filled with small print: you won't find pictures - no photos, no illustrations, no book covers, no maps. You won't find entries on specific science fiction characters - no page for Valentine Michael Smith - or aliens: look under Hal Clement and Larry Niven if you want anything at all on the Mesklinites and Puppeteers. And you won't find anything about what's been happening over the past decade, or more, in the field, as the book was last updated in the 1990s.
What you WILL get though is the best reference work covering the major authors, books, films, countries with science fiction writing traditions, comics, magazines, and themes and concepts in the field. The authors have said that neither it nor its equally indispensible "Fantasy" companion will be updated in print again -- they will be online only. That's a shame for paper-lovers like me; I cannot tell you how many hours I have spent in the company of this, paging back and forth between entries. Sure, clicking through a website is easier in some ways, but thus far that hasn't materialized. Virtually every writer and novel that I've ever heard of that could be considered part of the genre is in here; the authors seem to have missed nothing. Founding names in the field like Wells, Verne, Stapledon and Heinlein not surprisingly receive several page entries apiece, but it's rare to find any figure at all - at least any writing in English, I think the coverage outside of the language is a bit sparser - who doesn't get a fairly thorough bibliographic listing at the least.
One complaint that several reviewers of the Fantasy volume have made is the authors' usage of "made-up" terms like "polder" or "edifice" (not artificial words, but new in their application to the genre). I have no problem with this issue, as typically they've come up with language where none really existed before - they're filling a void - but if this is an issue for you, rest assured that it's less prevalent in this volume, perhaps because the terminology in the genre has been set down more systematically by academics and writers who have gone before.
You can probably get both this and its fantasy partner quite cheap now, and I would highly recommend them if you have any interest in the byways of the fields. One thing I really like is that neither book discriminates against the more "literary" figures (William S. Burroughs or Italo Calvino for example) who sometimes are given short shrift in genre-geeky references. The writing is generally serious, and I like the fact that "objectivity" isn't always the highest goal; no matter how popular a book or author may be, Clute and Nicholls are not afraid to cut them down to size, though never maliciously. And on the other hand they always seem to find something nice to say about even more marginal figures in the fiel. They are in short enthusiastic supporters of the genre as a whole, and I can't think of many writers who have done half as well at grasping the enormity of this gigantic and wonderful field of literature.