I agree with you about Wheen's treatment of Marx: he is a lightweight and isn't up to seriously examinining the ideas, but I also think you are a being a little unfair. This is not an intellectual biography and doesn't pretend to be one. I think there is room for this kind of writing. Not everyone wants to read an academic tome like McLellan's, for example, superb though it is. This book has attracted a wide readership and has done a great deal to clear away a lot of mistaken ideas about Marx (though, granted it preserves a few others as well). All in all, though, that must be to the good. I think Wheen needs to be congratulated for that.
I enjoyed it, even though it did irritate me too, at times.