The flavors combine somehow to make it VERY easy to drink. I need to try cointreau and a splash of coke to see if that really is the magic.

As yellow said, if it's properly mixed, the effect is to be a very sweet iced tea taste where the alcohol is nearly unnoticeable. It's probably due to the mixer/syrup just obliterating the harshness of the alcohol. Before I gave up drinking, it was definitely one of the few drinks that could sneak up and kick you in the ass if you weren't paying attention.

The fact of sweet mixed drinks getting into you faster likely is also a contributing factor, and the classic liquors that they use are 'sugar heavy.'

Depends on how it's made. I've had some (of varying strengths) where this was true, and others where the alcohol was more than apparent.

I've had margaritas like this, too. Unless it's really strong, in which case you're going to taste it no matter what.