I regard the first edition of 1977 as a classic commentary on Revelation. If you already own the first edition it is not really worth upgrading to the revised edition. For the scholar Mounce has been overtaken by the much larger Beale. However for the student Mounce is still one of the best commentaries available, I used Mounce more than any other commentary on Revelation I owned and I would recommend it.
Note: Among 7 recent scholarly commentaries on Revelation Mounce is the fourth most cited author.
End note: Since I wrote this review in 2000 (amazon.com) Osborne has been published and I would now put that at the top of my list for students. However, Mounce is a good deal simpler for those studying Revelation for the first time.