Rabbi Elli Fischer of ADDeRabbi has already reviewed the forthcoming The Koren Sacks Siddur (here), but I was sent a copy too, so I feel I should review it as well (which is, after all, why I received it).
Since Reb Elli dealt primarily with translation and it's treatment of women-friendly and Israel centered themes, I feel I should comment on other aspects.
First of all, it is beautiful. It so happens that I love the Koren font, but that is only part of it. It is layed out neatly, and the binding seems to be exceptional -- very important for a siddur that is to be used daily for many years. Elli already commented on the innovative move to place the Hebrew on the left and the English on the right. It does indeed take getting used to (say, a minute) but once you do it seems so complementary and natural. A good layout technique; my compliments to whomever thought of it. There are little quirky things which look nice; an asterisk marks the aliyos in the section of weekday Torah readings. A tiny sideways isosceles trinagle marks the point where the chazzan reads from. The English is in normal font, rather than italics (coughyouknowwhocough). From a layout-design perspective, the only complaints I have is that there is too little variation in font size. This isn't really a problem for someone who is comfortable with a siddur, but I can see this being difficult for some. Also, the section on berachos is not found after shacharis, as in most siddurim. Again, not a huge deal, but unless there is some significant improvement, changing the familiar layout should be reconsidered.
In the comments at Elli's review, someone suggested that the te'amim (cantillation signs) be included on all biblical verses. That may be extreme, but they are properly included for the Shema. However, I wondered why they were not included for Az Yashir; while it is a kabbalistic custom to say Az Yashir with the trope, this siddur does not eschew the kabbalistic influence on the siddur generally. Thus, all the expected le-shem yichuds are included, even if a little note that "some say" them precedes. I noticed, with my approval, that morid ha-gshm is pointed with segols.
I didn't get the chance to read the commentary and translation extensively (besides, Elli already commented on the quality of the translation), but I have a few words about it. British Chief Rabbi Jonathan Sacks's erudition and inspirational ability is known to all who have heard or read works by him. The little I did preview didn't reveal any earth-shattering commentary, but that is to be expected (preferred?) in what should be a popular and reliable siddur. I noticed that he had fun in his translation of the zemer Yah Ribbon Olam; in keeping with the rhyming Aramaic (ve-olmaya, malchaya, ve-simhaya, le-hachavaya, etc.) he rhymed all the stiches in English (adored, Lord, applaud, accord), keeping it true for all five paragraphs. Pretty cool. Upon request, I'll scan that page if anyone wants to judge how well he accomplished this.
I began to compile a list of sources cited in the commentary, but did not get very far. I noticed R. Joseph Ber Soloveitchik, R. Samson Raphael Hirsch, R. Jacob Zvi Mecklenburg, Ibn Ezra, Radak, R. Avraham Maimonides, and, of course, the siddur of Rav Amram, and Tolstoy. A great deal of the commentary seems to be original; at least that is the assumption I make for unattributed commentary. Since the edition is still not released, and improvements can yet be made, I'd like to go on record suggesting a complete bibliography at the end!
Finally, a little thing I noticed and thought was interesting. In Az Yashir (pg. 80, Ex. 15.3) we see יהוה איש מלחמה translated as "the Lord is a master of war." In the introduction to the Birnbaum siddur we find some of the principles which Birnbaum followed, and the reasons why he felt his siddur was a vast improvement over those which had come before him. We find the following example:
"Every student of Hebrew knows that בן is not always equivalent of a son. It frequently denoted age, membership in a definitive class, or the possession of some quality. Similarly, איש and בעל are often used interchangably to characterize a person. Thus, איש לשון, (Psalm 140.12) means a slanderer, and איש מלחמה, (Exodus 15.3) a warrior. Hence, the rendering "the Lord is a man of war" is erroneous and nothing short of sacrilegious." (Ha-siddur Ha-shalem; pg. xiv). Birnbaum goes on to call this type of "literalness . . . typical of what has crept into the Siddur's translation as a result of copying from men unfamiliar with idiomatic Hebrew."
Is R. Sacks's "master of war" the same as "man of war"? Maybe, maybe not. In any event, Birnbaum was not a native English speaker; Sacks is. Speaking of idiomatic English, I couldn't help but to think that "master of war" evokes - at least unconsciously - Bob Dylan's Masters of War. I can't imagine that Sacks wished to apply the message of that song to God; but I can imagine that the phrase was rolling around in his mind, just as it is in the mind of everyone who knows that song, and to him it seemed very good idiomatic English, that is, normally a phrase like that would seem stilted, but in this case "the Lord is a master of war" seems very natural to the reader.
In all, I like the siddur very much. I like how it looks, how durable it is, how relaxed with itself and its modern Orthodox ideology it is, how halachically normative it is, and that font! Love it. You also get R. Sacks's commentary on Pirke Avot free. You won't be disappointed if you buy it.