(I have the 2005 edition - I believe an updated one has been released since then.)
----What is it?----
I bought this out of curiosity, because it was cheap, and because I love science in general but have never had the time to read over much of it!
Having just received this I cannot give a full review (and being near 1000 pages it will be a long time until I can) but from what I've seen so far it seems excellent.
Personally, I'm convinced I will love this book. It's like a mini-Wikipedia full of things I'm interested in, but rather than each article being the length of a dozen pages of A4, each is small, concise, easy to understand, and to the point.
As others have stated, the cross-referencing is excellent. It's perfectly possible to jump into this book to find out about quantum mechanics, and come out very clued up on the Triassic period!
Finally, I've never bought a dictionary like this before, but this seems to take the word "dictionary" to an odd place. I'd say "Concise Encyclopaedia" would be better!
----Who's it for?----
Probably A-level or undergrad students, especially those interested in sciences they are not directly studying. Much of this content would be found on an undergrad course. This is not an A-level textbook, but would be approachable from A-level.
(This means that, to an average person, the first definition they seek could well be impenetrable without a few other definitions first... but then they find each of these just as impenetrable!)
And yes, some of the definitions, especially those in Physics, can get mathematical. This is to be expected: you can't publish a book aimed at students and sciencey people and neglect to mention the actual forms of Maxwell's equations, for instance. The publishers did the right thing, but I mention it because this may be intimidating for some people.
(I'll revise this review if and when I have used this product sufficiently, and if and when I remember.)