Seforim has a nice post on Chafetz Chaim's attitude toward Chassidus. I am not certain why R' Brodt left out the following excerpts from Dugma M'Darkei Avi which show the Chafetz Chaim to have been something of a Misnagid (although his opinion softened somewhat when he saw the Chassidim were a great ally in the struggle against Haskalah).
1 - "Who told you to got to the Shtiblach, I do not deal with them" (the Chassidim). After his son describes some incidents in which Chassidim had disparaged Lithuanian Gedolim.
2 - Against those who daven late with the excuse that it is necessary for proper Kavanos (classic Misnagdic complaint)
3 - Chassdim on the right, Haskalah on the left. Neither is a positive trend. Also, he was shown a Sefer of of the Admorim and said "What is missing in the Torah of the Tannoim and Amoraim" (i.e. who needs this new stuff?)
Although, it must be noted that his son was not a fan of Chassidim (as per story in 1) and is not an impartial observer.
I'd also note that the expressions "Gaon Echad", "Gadol HaDor Echad" and the like appear frequently in the Chafetz Chaim's Shem Olam. It woudl be interesting if someone can identify who this refers to and why the Chafetz Chaim chose to withhold his name.