Here's what Philip Towner writes in the preface:
There are, unfortunately, no shortcuts in the study of Greek or Hebrew if the goal is fluent reading. But in a time when tools for engaging the biblical texts are increasingly available and electronic solutions are dominating, the value of a resource such as "The UBS Greek New Testament: A Reader's Edition" may seem to require some measure of justification. In this case, the title explains behind this tool--reading the Greek New Testament. My own experience of reading, teaching and translating the Greek of the New Testament convinces me that familarity and "fluency" only come when the text can be read continuously and synthetically. But much of the focus in university and seminary teaching of the Greek New Testament today is on delivering to the students (by way of software and disjointed, abbreviated teaching strategies) analytical tools mainly designed to give access to dictionaries and commentaries. What is needed to ensure a next generation of scholars and translators of the Greek New Testament is the assurance that there will be those who can actually read the text without undue dependence upon "tools." And for this to happen, there must be a renewed emphasis today on reading the Greek text and concentration on developing resources which genuinely faciliate the acquisition of this skill.In conjunction with this, I'd highly recommend looking at the advice and schedule in Lee Irons's Annual Greek New Testament Program Using "The UBS Greek New Testament: A Reader’s Edition."
"The UBS Greek New Testament: A Reader's Edition" is just such a resource. This Reader's Greek New Testament has notable advantages over other similar tools. First, it employs the best available scholarly Greek text (the text of the UBS Greek New Testament which is identical with the text of the Nestle-Aland Novum Testamentum Graece). Then, it offers the reader who has a first-year level, working Greek vocabulary ready access to those words occurring thirty times or less in the Greek New Testament, as well as additional help with the identification of certain rare lexical and grammatical forms. Finally, the glosses provided aim to be contextually relevant to aid the student in the reading process. In short, the theory behind this tool is that provision of just the amount of information necessary for the reading task will aid the reader in developing naturalness in the skill of reading--including vocabulary building, mastery of syntax and familiarity with grammar. As reading skill increases, dependence upon interlinears and some kinds of software which hinder the acquisition of fluency can be minimized. It is something of a balancing act to find the lowest limit of the information needed to foster rather than impede uninteruppted reading. And "The UBS Greek New Testament: A Reader's Edition" has achieved that balanced nicely. This tool will help the reader to "graduate" to independent reading of the UBS Greek New Testament/Nestle-Aland Novum Testamentum Graece sooner rather than later.
The bottom line is this: to master the skill of reading the Greek New Testament there is simply no substitute for linear and sustained reading. This tool will aid the student and scholar in achieving this goal.
You'll note that Lee recommends A.T. Robertson's The Minister and His Greek New Testament--my link here takes you to a recent reprint edition.
One final note: some readers may wonder how the UBS Reader's Edition differs from Zondervan's A Reader's Greek New Testament. First, the Zondervan edition has a nicer (burgundy Italian duo-tone) cover, golden edges, with thinner paper and thinner margins. Zondervan's is 576 pages--a whole half-inch thinner than UBS's (which just over 700 pages due to a 100-page dictionary in the back). The Greek texts themselves are different (though not by much). Zondervan "reverse engineered" the NIV to produce the underlying eclectic text.
The first edition of Zondervan's (the one I own) has very odd typography: the entire thing is in italics, with OT quotes being in bold non-italic. But Greek italics is very hard to read. In addition, in the first edition all the footnotes (both the Greek word and the definitions!) were in italics. Thankfully the second edition rectifies that. (You can see sample pages here.)
Zondervan's has more glosses in the notes, though UBS provides parsing for the verb forms.
The UBS also provides English headings for the pericopes, along with cross-references in the headings of the synoptics. Some of the purists might not like this, but for a reader's edition I think it's a wonderful addition.
So there are pros and cons. But for me, the typography (not an insignificant factor in choosing a reader's edition!) makes UBS a clear winner. Even though the italicized Greek is gone in Zondervan's, I think the font is still much harder to read than UBS's. Further, UBS employs generous amounts of "white space" in the margins and between the text and notes. Finally, UBS uses a single-column format for the main text and double columns for the notes--which is extremely user-friendly. Zondervan's runs all the notes together. They had to do that, I'm sure, in order to keep the multiple definitions and keep the page count down. But the USB makes up for their single-word definitions by having the dictionary in the back.
So UBS is my recommendation.
Update: Well, if I had used a little-known tool called "Google" I would have discovered that in December 2007 Rick Mansfield beat me to the punch, posting a review with similar conclusions but with more detail and with pictures to boot!