I've just borrowed the book from a nearby library and will examine it over the next few weeks. The first thing I notice is that it weighs in at 547 pages including front and back matter. (I have the 1968 Oxford University press edition of Edwyn C. Hoskyns' 1933 translation.) Since the entire letter of Romans accounts for about a dozen pages, that's roughly 45 pages of commentary for each page of the original text. This is not a trivial or light commentary. (Nor is Romans an inconsequential letter.)
The blurb on the back suggests:
This remains a fundamental book for a full understanding of Barthianism.
Certainly, this must also be true of any Christian theologian who tackles Romans. But in his introduction to his English readers, Barth encourages us to as far as possible separate the content of this volume, which attempts to explain Paul's letter, from the context of its author. Barth is quite upfront about having added something of himself, as is inevitable when trying bring out the meaning of a complex text. To drive the point home, he somewhat distances himself from his commentary written 14 years before the translation. Many of the criticisms a modern reader might level had already been discussed by Barth and his German-speaking colleagues.
Barth also encourages us to read the entire work as it follows Paul's intertwined argument. This will present a problem for me since it's unlikely I will be able to read it all before returning it to the library. My hope is to use Barth's answers to questions on Romans, but I will need to be careful not to leave out important ideas presented in other areas of the book. This bring me to the first significant pro and con—two sides of the same coin:
Pro: Treats Romans as a complete, unified whole.
Con: Difficult to use in a piecemeal or verse-by-verse fashion.