Brackets & Nusach Sefard Siddurim
A Yid commenting on "What Siddur Do You Use?":
About your question on the sidur issue.
I personally use "Minchas Elozer" sidur compiled by the Munkatcher Rebe – R’ Tzvi Hersh Shapiro ztz"l (Baal Darkey Tshuvo) which was edited and completed by his son – R’ Chaim Elozer Shapiro ztz"l. (However I use it with some modifications, according to precise Kisvey Arizal). They worked to produce a Chasidic sidur according to nusach Ari za"l.
Minchas Elozer wrote a lengthy "Maymor nusach hatfilo" where he describes results of his work.
One main point here is that standard "Nusach Sfarad sidur" is a result of many sidur publishers and not a compilation of any tzadik or mekubol and therefore is very mebulbol. In short it was produces something like this: many tzadikim tried to compile Nusach Ariza"l sidur for themselves, and came with various results of it. Their chasidim followed their rebbes with this variations. Publishers (according to market demand) put all this variations in different brackets, which (brackets) with time were simply dropped! And that’s how most nusach sfarad sidurim came into being. That’s why for one who really wants to use Shaar haKoylel to the best of his ability, standard nusach Sfarad is not the best choice at all.
On the other hand, there was a need to produce a high quality sidur which is meduyek. And this was attempted by a number of tzadikim (like Baal haTania ztz"l). However here is another hidden problem, that is – a complicated history of how Kisvey Ariza"l became available in Europe. I shouldn’t write about it now, because it’s too lengthy and complex (if you want to understand it better look into kuntreys "Kisvoyni leDoyros – hishtalshelus Kisvey hoArizal" published by "Ahavas Sholoym". In short, they became available not all at once, but different parts in different times (separated by hundreds years). Therefore many tzadikim of the past didn’t have all ksovim while they compiled their sidurim. And the problem was back – to compile the sidur which is "up to date" according to all available Kisvey Arizal. That’s what Minchas Elozer deed. (He lived not so long ago – just before the WW II).
Now a bit about R’ Rimmer’s Tfilas Chaim sidur. While I value the work of Reb Daniel Rimmer (he wrote an extensive article about many aspects of the nusach Arizal from many perspectives, and collected many sources as footnotes to the sidur showing real nusachoys from Kisvey Ari), but I don't necessarily agree to his conclusions. He discards the method of talmidey Baal Shem Tov and the Maggid to apply shaar hakoylel as a modification of nusach Ashkenaz according to Ariza"l. He even questions if they really meant it (even though in several places he explicitly says that they did, by this contradicting his own sfeykoys). Therefore I doubt that many chasidim will use his sidur practically for tfilo, but of course it is very precious as an extensive reference, nice presentation of nusach research and study manual on this subject.