I suggest looking at A Stillness at Appomattox by Bruce Catton, from the Army of the Potomac series. Catton relied extensively on regimental records, and the letters written by soldiers in the ranks. You will be able to find a lot of choice quotes in there. Obviously, their opinions of Grant varied with the fortunes of war, but generally the soldiers under Grant accorded him with quiet respect, as someone who could get things done, and wouldn't let their efforts go to waste by bungling command decisions. This steadfastness was particularly appreciated in the Army of the Potomac, which had suffered though its share of inept generalship.