I have about 10 of these things around the garage and shop. I keep one hooked up for each item that I have that (pretty much) has a battery....
The boat, lawn mower, 911, E23, E38, F250... The Mustang has a special one that is for AGM batteries... I have had great luck with them in keeping the batteries going while whatever they are attached to sits.... and sometimes sits... and sits...
I have a mix of hookups: clip, hard mounted and cig lighter.
Like what Terry said.. driving off with one of these hooked up would be not so good. So - for the clip and hard mounted ones - I drape the cord over the side mirror so I will see it before I drive off.
Battery Tender is also ok. I have just had good luck with the BatteryMinder version.