I jumped into the M12 platform with the 2401 screwdriver and the underhood light (2125 I think it was). The quality of the tools seems to be good, but I had some issues with the XC4.0 battery that came with the underhood light.
Both kits came with what I'd assume is the standard M12 battery charger. Just charges a single M12 battery. Both the 1.5ah batteries that came with the screwdriver charged up fine, although having experience with the DeWalt 20v system, the attachment of the batteries to the charger doesn't really feel that positive or confidence inspiring. No nice "click" feel on engagement.
Anyways, the XC4.0 I plugged into the charger it came with and immediately got simultaneous flashing red and green lights. Tried it in the other charger and same thing. Took the kit back to HD and exchanged it for another assuming the battery was pooched.
Got home with the new one and that 4.0 was doing the same thing. Started comparing the 1.5's in the chargers to the 4.0 and the 4.0 didn't seem to be going as deep. Pushed hard and still wouldn't go more. Pushed hard and wiggled and still no go, just red and green flashing.
I didn't want to look like a nut going back to HD again so I pushed it in and wiggled even harder, like borderline felt like it was going to break, and finally popped in fully and started to charge. Took it in and out a few times and it's still very hard but not quite as hard as the first time.
Not a big issue, but seems like a batch of XC4.0's where the terminals aren't quite right. That plus the lack of positive feedback on actually getting the battery fully in makes me feel a lot less positive about these than I thought I would. The DeWalt 20v batteries clip right into the charger the same was they snap into the tools, and I'm not sure why Milwaukee couldn't have used the small tabs on the sides of the batteries to give a positive engagement into the chargers too. Seems like a corner being cut, but since the tools themselves are nice I'll give them the benefit of the doubt.