GMAT OG Quant Review, 2nd Ed: Problem Solving
Like Data Sufficiency, Problem Solving hardly changed from the 1st edition to the 2nd edition.
Qualitatively, the new problems are very similar to their predecessors, testing fundamentally identical content. The phrasing or some other minor aspect may be original, and in general, the new problems are excellent additional examples of GMAT-like style and feel. So these problems are absolutely worth doing. Just realize that they are essentially "more of the same."
As far as topic distribution goes, there are more Geometry problems and slightly fewer Fractions, Decimals, & Percents problems than before. However, the overall distribution of topics has not changed dramatically.
The 1st edition had 176 Problem Solving problems. 45 problems (26%) were removed, leaving 131 repeats. Then 45 problems were added, yielding 176 problems—the same number as before.
The proportion of problems in various topical categories has shifted, but only a little. Geometry has expanded somewhat, but at 11% of the whole, it’s still the runt of the litter. Likewise, Fractions, Decimals, & Percents has shrunk somewhat, but it still constitutes a quarter of all problems. Do not over-interpret these changes.
Problems were removed from the 1st edition at all levels of difficulty (as measured by problem number), but unevenly. Significantly more problems were removed from the top and from the lower-middle than from the bottom and from the upper-middle.
Meanwhile, problems were added in generally the same pattern: many more in quartiles 1 and 3 than in 2 and 4. Repeated problems did not change very much in number; in fact, many problems have identical numbers in both editions. This indicates that the additions and removals happened in roughly the same places in the sequence.
The average difficulty for various topics generally remained the same. For Geometry, the average difficulty rose significantly with the addition of several higher-level problems in this area.