I used to pray the Shemona Esre {a fixed prayer in the Jewish Prayer-book} every day three times with a lot of fervor. The fact that it was fixed did not stop me from deep intentions. Private chatting with G-d was something I started later I but I did not think my intentions in these private chats were any more deep or sincere. They can be. But the fact that they were spontaneous did nothing to add to sincerity.

The most deep aspect of prayer was when I was in Shar Yashuv [that is Reb Freifeld's yeshiva] in Far Rockaway and the Mir Yeshiva in NY.

Private chat I think is a good thing but it should not be confused with sincerity or depth.

Later the prayer-book prayers kind of became dry to me and I added the intentions of the יסוד ושורש העבודה and Yaakov Emden in his great Sidur. [Reb Freifeld gave me that Sidur]. Sometime in Israel Elise Meir's brother sent to me the Sidur of the Reshash. [The red one]. And I started using that. {I had already been learning the Ari's books for some years.] But I always longed to get the large Sidur HaReshash by the grandson of the Reshash for a few reasons. Mordechi Sharabi said there were errors in the small sidur. Eventually I found the large sidur in the end of Mea Shearim near Rechov Salant. [Anyway, I have no idea what Mordechai Sharabi was talking about. The two sidurim are different systems. That is about it.That is they are two different interpretation of the Nahar Shalom of Shalom Sharabi. But mistakes? If he said so fine, but I did not see any.

When I pray today I try to stick with the basic, straight, Ashkenazic Prayer-book. The reason I stopped the Sidur with the intentions of the Ari is simple. The intentions are vessels for the light. Light changes so do the intentions. 

Still if you can get either the large or small sidur of the Reshash both are great books--if you read them along with the book of the Reshash the Nahar Shalom

[I am not so much into this anymore, But I do think the Ari and the intentions have great value when done right.]
What I think today is that there is nothing like straight prayer in a regular Litvak Yeshiva.