I really like the Alemite cordless gun, and believe they make about the best manual ones, too. I used an Alemite for 9 years and had no problems with it, other than buying one new set of batteries.
My Alemite still works, but I just bought an M18 Milwaukee this fall since I carry 2 impacts, a grinder, drill, ratchet, and sawzall of theirs. The M18 is good, and the lithium-ion batteries are far and away better than the ni-cads I was using, but, priming and purging the air from the M18 is nowhere near as easy and convenient as the Alemite is. The Milwaukee is a good bit heavier and bulkier, too.
All in all, if I was going to just use a cordless grease gun, it'd be an Alemite, since they use Li-ion batteries now too. If I wanted it to fit in with other cordless tools, it'd be the Milwaukee.