Having purchased both the M12 drill and Hackzall kits, I've had 3 of these chargers in my posession. Only 1 worked perfectly out of the package. The first charger (with the drill kit), thought both battery packs were defective. Amazon sent me a new kit - with it's charger, all battery packs charged fine. Wanting more batteries and another charger, I ordered the Hackzall kit. Inserting the first battery to charge again lit the faulty pack lights. Looking into the charging well, one of the extra 3 sensor contacts was bent down flat on the bottom of the well. Bending it back got the charger working. While it's great that it was a simple problem, a factory fresh charger should not have a problem with functioning.
When I took the charger with the bent contact apart (T10 security bit), the inside of the charger seemed well designed. The case seems to be made of polycarbonate and the circuit boards are the high quality glass variety. There is no potting compound over the circuit parts like in the V18/28 chargers though. Also, the plug on the M12 charger is not a pass through like on the V18/28 chargers.
If Milwaukee sorts out the bugs and sends you a working charger, it's probably a 5 star device (it even has slots on the back for wall mounting!).