Linda Belleville's II Corinthians commentary in the IVP New Testament Commentary series is one of the best of the briefer treatements of this important but often underemphasized letter. I would say that the only close rival at this level of detail is Scott Hafemann's NIVAC, which has the advantage of being a little later (and thus could benefit from Belleville's work and the scholarship since) and with more emphasis on applicational issues, whereas Belleville has a little more focus on the exegesis itself.
I like Belleville's approach for the most part. She seems to me to be much more balanced than some of the more detailed academic commentaries. She argues for Pauline authorship of the entire letter. She tends to favor seeing the letter as a unity, with some caution that certainty is impossible. She finds no absolutely convincing explanation about why the last few chapters seem very different, but she nonetheless does not take the differences to demonstrate their being taken from some other letter, and we should give the letter in its current form the benefit of the doubt in the absence of clear evidence. Overall, her exposition captures well the basic themes of this letter and how it ties in with I Corinthians and demonstrates both a familiarity with the literature on the epistle and an eye for how to interpret and apply its message in our contemporary setting.
Her goal seems to be to provide enough information from the best scholarship on the book to understand what Paul is up to in this letter without getting too bogged down in some of the more thorny problems. Sometimes she just refers to other scholarship when the details are tricky and the importance of the disagreement is less significant. She does give exegetical and text-critical notes at the bottom of the page, with a running exposition (not always verse by verse) taking up most of the space on most pages. It's hard to read both in order, however, since she does not use the notes at the bottom as footnotes. Since the commentary proper is not always verse-by-verse, it's difficult to figure out when to read the notes at the bottom. Other than that, the commentary does not seem like a scholarly reference work but feels like a book you can read.
For detailed scholarly work on this book on the level of the Greek text, try Murray J. Harris' NIGTC, and for a more detailed exegetical work without the detailed Greek I recommend the NAC by David Garland. But for this level of detail Belleville is excellent.