The notes in this edition were excellent for a quick comprehension of Seneca--both his general context and specific business with this tragic cycle. The notes were never sparing of historical and mythic context, though often when I found some difficulty with the text itself, whether syntax, vocabulary or grammar, they did little to assist me. I recommend you find an English Phaedra for the sore spots in your reading. The editors took no caution to hide their disfavor for the play. An example, 1267n: "This is arguable the worst line in Seneca drama." There are many other moments like this one, include several notes that contrast the Phaedra as a weak counterpart to Euripides' Hippolytus. The play may, indeed, be terrible, but why not let the reader see that themselves?
Overall, this edition presents a clear introduction, readable, compact notes and a careful, sightly presentation of the text. My quibbles are small, not substantial.
APPENDIX: On Ahl's Phaedra
I'm not sure why Amazon decided to loop together the reviews for Ahl's translation and Mayer/Coffey's commentary, but, by coincidence, I also made use of Ahl, so I'll offer a short review.
I used Ahl as an aid to translation, which was a mistake. His translation is clearly (and masterfully) designed for performance, not scholarship. Ahl works wonders with English prose, but these wonders often fly away from the Latin itself. Ahl admits to this, and I don't insult him for doing so. I would love to see his Phaedra performed. That said, because he plays with line numbers and tends to elaborate and clarify by adding to the Latin, the translation is poorly assembled for assisting a Latin reading.
APPENDIX SECUNDUS: On Mayer's Duckworth Commentary
Oh hell, it looks like Amazon has decided that every book on the Phaedra is the same book. Here, I'll offer my impression on Mayer's Commentary on the Phaedra, published in the Duckworth series. Here, Mayer is a bit lighter in his criticism of the play itself (perhaps it was Coffey who carried all critical weight in the Latin text commentary), but still offers consistent and careful criticism of various later performances and translations. His words are careful, but frank. Of course, the commentary also attempts to offer context and present various uncertainties about Senecan tragedy (performance, Stoicism, political context, etc.). Overall, it's a nice--and brief--companion to the Cambridge Latin text and commentary.