Starts off strong, then flags (progressively) after about the halfway mark.
Some of my impression may be due to personal bias in favor of Asimov essays wherein he explains scientific, mathematical, or historical fact; essays of this type comprise the first part of this collection ("Concerning the More or Less Known", about half of the book's total length).
All of the essays in this part are solid; they are divided into two sections, "Life", consisting of nine essays on mostly biochemical subjects, and "Nonlife", ten on inorganic chemistry, physics, and astronomy. The opening two entries provide an intriguing look at what must have been close to the state of the art in neuropsychology and genetics at the time. These essays pretty closely resemble Asimov's essays for The Magazine of Fantasy and Science Fiction at the time, and as I enjoy those tremendously, I found them comfortable and illuminating.
[Aside: I should note in particular Chapter 13, "Our Evolving Atmosphere" (am I the only person for whom such titles resurrect horrific memories of primary school textbook pabulum?), published in 1966 for the science annual of the World Book Encyclopedia spoke frankly about the greenhouse-gas consequences of massive carbon dioxide releases into the atmosphere as a result of human activities. I mention this because Asimov is popularly derided in the online global warming "debate" as having been a "global cooling" doomsayer in the 1970s.]
By contrast, the second part ("Concerning the More or Less Unknown") is a collection of more speculative fare. Its first section is "Other Life", which is tolerable, but the title essay of the book, and best entry in the section, "Is Anyone There?", treats techniques of communicating with extraterrestrial civilizations and suffers by comparison to Carl Sagan's later work on the same subject. (For the curious, the two radio sources, CTA-21 and CTA-102, for which Asimov held out hopes--"highly unlikely" ones, as he put it--as beacons constructed by extraterrestrial intelligences appear to have faded almost completely in notoriety, but continue to be studied as examples of "Gigahertz Peaked Spectrum" radio sources.)
The second half of part two, "Future Life", is, I'm sad to say, mostly a fairly dull effort at prognostication. Some of that may be inevitable, though, as the first two essays, "The World of 1990" and "The World's Fair of 2014", are inescapably going to read dramatically differently to a person in 2009 than to Asimov's original audience (or even the one that picked up "This Ace Printing: August 1980", per the copyright page). The last essay in the section, the prosaically titled "The Universe and the Future", is one of the longest essays in the book and is an extended consideration of what it will mean to undertake human--and post-human--exploration of the cosmos given currently-known limitations. The worst thing about the essay is the author's coinage of the ugly term "spome" (whose etymology he deliberately withholds until near the end), and while this is a superficial gripe, the ubiquity of its use makes it grate. As a significantly positive counterbalancing factor, Asimov does not even mention superluminal travel, wormholes, or hyperspace, and his omission of these currently-thought-impossible technologies consequences on interstellar exploration are sobering but strangely fresh, given that science fiction--at least the proletarian varieties within my limited horizons--tend to stipulate one or more of these (or, like Kim Stanley Robinson's Mars Trilogy, are set near enough to us in time and space that they need not be confabulated).
The final six essays (of thirty-seven total) compose the final part, "Concerning Science Fiction", and are the weakest of all, with the penultimate and antipenultimate entries being tongue-in-cheek reviews of then-contemporary television science fiction programming. That said, the last essay "The Lovely Lost Landscapes of Luna" is more thoughtful and somewhat redemptive--as Asimov grew from youth to middle age, from avid reader of Burroughs's John Carter of Mars books to scientifically-educated follower of the Mariner program, he watched the remote plausibility of early SF evaporate altogether, and the essay serves as a kind of eulogy for an age of lost innocence and (blissful?) ignorance. Asimov's enthusiasm for actual science fact, so effectively communicated in the first part of the book, contextualizes his wistfulness and makes it palatable to readers who (like Asimov) are easily irritated by know-nothingism (also see Chapter 33, "The Cult of Ignorance").
Bottom line: If you're a big, big fan of Asimov's nonfiction, read the first half and possibly chapters 31 and 37. Non-Asimovophiles will probably want to skip this entirely.
Contents:
I. Concerning the More or Less Known
A. Life
1. Matter over Mind ("That Odd Chemical Complex, The Human Mind"; The New York Times Magazine, 1966-07-03)
2. I Remember, I Remember ("Pills to Help Us Remember?"; The New York Times Magazine, 1966-10-09)
3. The Hungry People (Mademoiselle, 1960-10)
4. Blood Will Tell (Think, 1962-02)
5. The Chemical You (Mademoiselle, 1963-01)
6. Survival of the Molecular Fittest ("The New Enzymology", Consultant, 1965-05)
7. Enzymes and Metaphor (Journal of Chemical Education, 1959-11)
8. A Pinch of Life (Science World, 1957-03-05)
9. Constructing a Man ("Conceived in the Love Bed of Science"; True, 1966-02)
B. Nonlife
10. The Flaming Element (Petroleum Today, Winter 1961/1962)
11. Let There Be a New Light (no original publication information)
12. The Ocean Mine (Science World, 1957-03-19)
13. Our Evolving Atmosphere (Science Year--The World Book Science Annual, 1966)
14. The Atmosphere of the Moon (Venture Science Fiction, 1958-03)
15. Man and the Sun (no original publication information)
16. The Unused Stars (Amazing Stories, 1959-07)
17. Measuring Rods in Space (Space World 1961-09)
18. Time-Travel: One-Way (The North American Review, Summer 1964)
19. The Birth and Death of the Universe ("Over the Edge of the Universe"; Harper's Magazine, 1967-03)
II. Concerning the More or Less Unknown
A. Other Life
20. A Science in Search of a Subject (The New York Times Magazine, 1965-05-23)
21. We, the In-Betweens (Mademoiselle, 1961-05)
22. Is Anyone There? ("Hello, CTA-21--Is Anyone There?"; The New York Times Magazine, 1964-11-29)
23. Anatomy of a Martian ("Anatomy of a Man from Mars"; Esquire Magazine, 1965-09)
24. On Flying Saucers (original to this collection)
B. Future Life
25. The World of 1990 (The Diners' Club Magazine, 1965-01)
26. The World's Fair of 2014 ("Visit to the World's Fair of 2014", The New York Times Magazine, 1964-08-16)
27. Fecundity Limited (Venture Science Fiction, 1958-01)
28. The Price of Life (Cavalier, 1967-01)
29. The Moon and the Future ("What Can We Expect of the Moon?"; The American Legion Magazine, 1965-03)
30. The Solar System and the Future ("How Far Will We Go in Space?"; The World Book Year Book, 1966)
31. The Universe and the Future ("There's No Place Like Spome"; Atmosphere in Space Cabins and Closed Environments, 1966)
III. Concerning Science Fiction
32. Escape into Reality (The Humanist, 1957-11/12)
33. The Cult of Ignorance ("The By-Product of Science Fiction"; Chemical and Engineering News, 1956-08-13)
34. The Sword of Achilles (Bulletin of the Atomic Scientists, 1963-11)
35. How Not to Build a Robot ("Why I Wouldn't Have Done It This Way"; TV Guide, 1965-01-16)
36. The Insidious Uncle Martin ("Can You Spot the Family Resemblance?"; TV Guide, 1966-03-05)
37. The Lovely Lost Landscapes of Luna (P.S., 1966-04)