Alec Motyer writes an excellent and sensible commentary on the text of Isaiah. He is conservative on the unity of the book but does not waste large amounts of space dealing with unnecessary critical issues, (something that is thankfully welcome in a world where the unfortunate scholarly trend is to mangle the text with source and form criticism). He is also a sane interpreter of prophecy, and doesn't press the language of the text into absurd "over-literal" meanings. In short, Motyer gets high marks for writing a commentary that reads the text on its own terms.
Motyer's exegesis of the text mostly consists of analyzing individual words and showing parallels, both with other parts of Isaiah and with other portions of Scripture. He does not include a translation of the text, so you'll need to have a Bible open while reading this to follow his points (although if you're reading any commentary, you should really be doing that anyways!) I found a number of his literary considerations to be helpful. He also takes the New Testament use of Isaiah seriously, which adds an additional level of insight to the text.
The only criticism I have is that sometimes Motyer's outlines of specific passages strike me as arbitrarily contrived. It is as if he is trying too hard to make parallels and chiasms "fit" with each other. For instance, in the section on the oracles against the nations (chs. 13-27), he argues that the section breaks into three cycles (13-20, 21-23, 24-27), but then tries to make each individual element in the cycle parallel to each other, resulting in some rather awkward parallels. (One wonders, for instance, how what the oracles against Moab and Arabia have to do with each other, and furthermore, how they would be parallel to the "banquet" passage in 25:1-12). I am also not sure I agree with his overall three-fold division of Isaiah into: "The Book of the King" (1-37), "The Book of the Servant" (38-55), and "The Book of the Anointed Conqueror" (56-66). However, these concerns are really secondary compared to the value of this 500+ page work.
Overall, I would still recommend Motyer's commentary. He has a number of useful exegetical insights. If I could only have one Isaiah commentary, Oswalt would be my first choice. If I could have a second, I would get Motyer and use him to supplement Oswalt.