This article gives a pretty detailed review of the new Rabinical Council of America Siddur, which is something I hope to add to my bookshelf if it ever becomes available in an accessible format.
I have the Artscroll weekday Siddur on my phone as an app, and while I love how the app displays additions smartly, I find the translation to be too slavishly literal to the Hebrew, (it goes so far as to follow Hebrew grammar in translation, which results in unreadable English), even despite the terms on which the Artscroll siddurim are translated: specifically that Hebrew is a holy language and that since Hebrew is not understood by a lot of people they should be able to pray in another language with the translation reflecting as much of the Hebrew syntax as possible.
Another problem I’ve found, (and I’m not sure if this is a VoiceOver problem or a problem with the app itself), is that if your operating system language is in English, even if you have Hebrew braille tables installed, the app will not display Hebrew text properly, which means that I have to keep the app in English and scroll past the mangled Hebrew text where it appears.
I haven’t tried switching my VoiceOver language to Hebrew yet to see if it corrects the problem.
But to get back to the RCA siddur, I’d like to read the commentary and essays that come as part of the package.
There was talk during the early years of the siddur’s preparation of an online resource which contained the commentary and essays, but as far as I know it’s never come to fruition. I’ll echo this author in stating that it would be a areally good idea.