You can only watch one HD channel per home at a time. If the living room TV is tuned to an HD channel none of the other 3 TV's can be watching anything in HD. If the DVR is recording an HD program non of the other TV's can tune to an HD channel and get HD resolution. In fact you will get an error if you go to, say TNT HD, that telsl you to tune to the regular TNT channel.
In reading reviews online (which I would recommend you do) it also seems that HD on the U-Verse come in at a 10MHz MPEG4 stream while Dish and DirecTv both send a 12-14MHz stream. This will be noticable if you A/B a larger TV. If the highest possible definition is important to you keep it in mind, if you are like mose people and won't notice this don't worry about it.