While the opponents of Communism who have reviewed Marx on this site make strong points, they unfortunately do not respond specifically to this work. Communism may be outmoded, outdated, and even dangerous, but the fact remains that Marx's COMMUNIST MANIFESTO is an important document in the fields of economic and political history. The ripple effects of this work can still be seen today. A 150 year-old document which still has such significance to today's world should not be so easily dismissed. Instead, in the interests of a fair evaluation of this work, we must judge it in light of its context and its intent.
Marx's COMMUNIST MANIFESTO is, as has been mentioned in other posts, a work of propaganda. As such, it is much lighter reading than some of Marx's other works (e.g. Das Kapital). It was originally intended for industrial workers and was written to be understood by them. The style is equally accessible to the modern reader. As many have said in previous posts, if you are looking for a concise, readable summary of socialist/communist ideology, this is probably your best bet. If you are interested in a more heavy-duty discussion of the Communist world-view, you might want to consider reading one of Marx and Engels' bulky theoretical works.
You wouldn't know it from some of the posts on this page, but the COMMUNIST MANIFESTO has had both positive and negative consequences. Attributing Stalin's slaughters solely to Marx, for example, is certainly unfair. Many factors, not the least of which was Stalin himself, led to those tragic events. Similar points could be made concerning other abusive "communist/socialist" regimes.
We must also consider the positive effects this work has had on the world we live in. When Marx wrote this pamphlet, the industrial revolution in England was causing tremendous suffering and injustice. Laissez-faire capitalism had created a very unequal system. Those of you familiar with the turn-of-the-century United States probably recognize this scenario. The form of democracy at the time of this book's introduction closely resembled an oligarchy, in which the rich and powerful ruled over the weak. The impact of socialist ideology on this situation was very great: labor movements were created, egalitarianism became a greater part of democratic ideology, and the lower classes became more significant to the political system than they had ever been before. In short, a significant part of what we call "democracy" can be indirectly traced to Marx.
For the record, the greatest weakness of Marx's argument, in my view, is his failure to predict the significance of the middle class in the wealthiest nations. Marx's view was that the middle classes would either be absorbed into the working class or would become proprietors (i.e. bourgeois). The success of the middle class probably accounts for the failure of Marx's predictive theory.
The irony of Marx's ideology, of course, is that Marx himself (and Engels, for that matter) was a member of the upper class (or, at the very least, of the upper middle class). He never experienced the misery which he opposed. Nevertheless, for a straightforward, clear articulation of a worldview that rocked the world, this is the book to read.