In terms of tone and style, The Joy of $x$ is an absolute delight -- Strogatz has a knack for finding the right analogy and the right anecdote that is the envy of maths writers everywhere. It's an enjoyable read, and I'd recommend it to anyone who thinks "I'd love to know more about maths, but I don't really know any."
Which, when it comes down to it, is my main problem with it. I understand: I'm not the target audience. All the same, I like my maths books to teach me something interesting I didn't already know. This is a steady, straight-down-the-middle explanation of the concepts of maths, literally "from one to infinity", covering things like commutativity, quadratics, conics, differential equations and even1 stats.
It's a nice book, a good gift for an interested party, and fun even for a know-it-all like me, but I'm afraid it's not going to win the Flying Colours Prize for Being an Interesting Book: while it scores highly in the "light and entertaining" category, it lets itself down in the "oo! I never knew that!" stakes.