I will get flamed for this but i don't like the battery powered grease guns and we have a metabo and dewalt hanging on our bench wall.
I rarely go through more than 2 tubes at any one point in time so not a heavy grease user per session although i do buy grease by the case.
Why don't I like? I think it takes away the art (feel) of greasing. I loose the feel (pressure) and the sound. By hand I can feel when resistance starts to build and stop pumping before blowing out seals, destroying bearings, etc... Also by hand I can hear the grease through some fittings / seals.
So for me i am not prepared to give up art of having the feel and sound and knowing the grease is flowing, when resistance is felt, easier to avoid overfilling / blowing out seals, destroying bearings, etc.... I am prepared to go slow, take time, have better feel for greasing tasks. i have no need for NASCAR speed during grease servicing.
but heck that's just me and I rarely go though more than 2 tubes at one time so the old / cheap pistol grip works fine for me. But options are always good i suppose.
edit: also as someone just told me this morning, by hand makes it easy to tell if zerk is plugged or taking grease helps avoid letting joint go dry.