I prayed with the sidur of the Reshash for many years. That is the small red one in three volumes, and then around the end of my second time in Israel, I bought the large one from the grandson of the Reshash which was being sold privately in Mea Shearim for some ridiculously low price. I forget offhand what the advantage was in the large sidur except that it was more complete. Vaguely I recall the big one had the intentions of the Omer and the Hagada of Pesach.

The main advantage of using the sidur of the Reshash is mainly if one has already the Divine light shining on himself. It does not bring down the Divine Light, but can only channel it.

In any case, at some point I decided once I had pushed off the Divine Light (for reasons known to me) to stop using it because, after all, what was the point? It just made myself more visible to the Sitra Achra.

I should mention that it is not at all obvious what the Reshash is all about until you read דרוש הדעת and you get to the end of the עץ חיים (Tree Of Life) of the Ari. [That is the last 1/4.] That is where he starts to modify his system. Up until that point, everything looks fairly simple. But that is where he starts to include the worlds one in the other, and that highly modifies the whole system. Still, the more basic ways of understanding the Ari which come from Rav Yaakov Abuchazteira [grandfather of Bava Sali], the Gra, and the Ramchal seem to me to be perfectly fine as far as the simple explanation goes.

I have to issue the usual warning however, that to ignore the signature  of the Gra on the letter of excommunication is to invite the Dark Side. And inviting the Dark Side, is like inviting Hell's Angels into your home. Once they are there, you can not get them out.