I found it effective when I studied the last 40 or 50 ds problems. I became a little bit bored with the first couple hundred. I missed quite a few of the last problems in practice in spite of getting a 49 math. Personally, I thought that some of these problems provided the right type of practice with numbers properties to break apart tough ds problems. They even helped me on tough ps problems.
Remember, if any problem begins taking you longer than two minutes, you might not be approaching it correctly. Learning the "proper" approach is applicable to all kinds of questions you'll see on the real thing. Studying the toughest problems intently let's you know the types of concepts that will be tested and the "shortcuts" to answering what appear to be complex compound problems. That's much more important than studying volume. Instead study efficiency.
Sorry for all the quotation marks. I think they add emphasis.