While an oversimplification, the book is a kind of an instruction book for the rulers (i.e. princes) of 16th century of city states and other such smallish states.
According to Machiavelli, the primary goal of a prince is to obtain state rule, and once ruling, keep ruling. The methods put forth are to be judged only in terms of whether those are effective towards the ultimate goals of obtaining and maintaining state rule.
The book is notable in that it abandons 'ideals' in favor of 'what works', apparently an innovative choice in 16th century political philosophic writing.
Was it good?
Well, I found some sections of the book significantly more rewarding to read than others. In a nutshell, parts 7 and 8 (chapters 14 through 23) are in my opinion whether the most characteristically Machiavellian stuff resides. And some of that stuff is quite straightforwardly applicable even today: for example, Machiavelli underscores that only appearances matter, regardless of what actually would be the case:
"He should appear to be compassionate, faithful to his word, guileless, and devout. And indeed he should be so. But his disposition should be such that, if he needs to be the opposite, he knows how.” (chapter 18).
The main take-away for me?
Again, the main take-away perhaps resides at the meta level. On the one hand, Machiavelli is very realistic in discussing 'what works', but on the other I'm a big believer in 'how things should be' (both descriptively and normatively). Thus, the take-away is some kind of a lingering background thinking about the mutual relationship and respective worth of each of these perspectives.
Who should read the book?
I wouldn't recommend the book as such, because it is quite tiring at places for a modern-day reader. Instead, I would recommend some kind of an introductory text in political philosophy where Machiavelli would be discussed and put into context. For example, Yale University offers an open course Introduction to Political Philosophy, with two lectures devoted on Machiavelli -- and this book in fact.
The book on Amazon.com: The Prince