Brad De Long makes a couple of good points about reading Marx. But let's be clear - they do not invalidate Marxism.
Point 1 - it's just silly to read Marx "line by line, paragraph by paragraph (at least in the early chapters), discussing and arguing over every page."
True. To get the most out of Capital vol I - for Christ's sake don't call it Das Kapital, unless you're in the habit of calling all foreign language books by their original title - you should read it from the middle.
Start with chapters 10-15. Then read part 8. Only then, start at the beginning, if you must. You'll learn the following, among other things:
1. Marx was a great empiricist; the chapters are crammed with facts about the 19th C economy and living conditions. The idea that he's an idle Hegelian arm-waving theorist is plain wrong.
2. Marx was a humanitarian. His hostility to capitalism (which was of course tempered by an admiration of its impact on productivity) was based upon outrage at the barbaric impact it had upon working people.
3. Marx recognized, following Smith, that capitalism's high productivity was based upon co-operation. The question he asked was: how could bosses, rather than workers, seize the fruits of this co-operation? His use of the labour theory of value was just one attempt to answer this question.
4. Capitalism did not emerge from the voluntary transactions of free people. It was founded upon state-sanctioned theft and oppression.
Point 2 - "If you try to ground an analysis of capitalism-in-particular on a feature (the distinction between objects' direct usefulness and their role in social processes of reciprocity, redistribution, or market exchange) that capitalism shares with every other human social system--well, you won't get anywhere."
This too is true. But fretting about the distinction between use-value and exchange value did not start with Marx. Take this:
The word value, it is to be observed, has two different meanings, and sometimes expresses the utility of some particular object, and sometimes the power of purchasing other goods which the possession of that object conveys. The one may be called "value in use"; the other, "value in exchange." The things which have the greatest value in use have frequently little or no value in exchange; and, on the contrary, those which have the greatest value in exchange have frequently little or no value in use.
It's Adam Smith (book 1 ch 4 of Wealth of Nations).
And this raises a point that really irritates me. Why is it that so many people think Marx was an idiot for subscribing to the labour theory of value, whilst Smith was a genius even though he subscribed to an even naiver version of that theory?
But then I know the answer, don't I? Smith's in the right tribe, and Marx is in the wrong one. The fact that both men had great insights, and flaws, is irrelevant. Because what they actually wrote isn't the point, is it?