I wanted a charger to keep my car battery charged (but not overcharged!) while I am absent on multi-week trips. So far the Duracell 2 Amp charger seems to be working well.
It is a three-stage charger. The specification says that depending on the current state of the battery, it will do a fast charge to around 80%, then a slower charge to almost full, then finally move to a lower voltage "floating" charge to keep the battery near 100% without overcharging.
I used a multimeter to watch the charging process. My battery was mostly full to start with, so the charger started charging at 14.1 volts (the slow charge mode) and then after a few hours moved to 12.7 volts (for the floating charge). These both seem appropriate voltages. In particular, the floating voltage of 12.7 volts should be safe for long-term battery maintenance.
After about two days on floating charge, my charger's red charging light finally went off. Over the next couple of weeks, when I checked the charger, the red charging light was mostly off but I occasionally noticed it on. It looks like the charger is moving between off and floating charge as needed for sustained maintenance.
I'll have to see how the charger holds up under long-term use, but so far everything looks good.
Update November 2010: I've been using this for two years now and I continue to be very happy with it. It's pleasant to come back from a four week absence to a fully charged car battery, with zero hassles!