Personally I use the clips to attach, but would prefer to solder. I'm just not confident in my soldering skills. I use a dab of silicone and black electric tape to secure the points of contact after pinching the clips shut, and then use aluminum tape to secure the heat tape to the back or bottom of my shelves. I have used both methods of placement, and the benefit of putting it on the back of the unit is that the containers don't rough up the tape if it's on the bottome of the shelf. As for output, I have two sections of 11" x 4' heat tape on a 4' x 2' shelf unit. I have a dimmer inline to control heat with digital indoor/outdoor thermometers and an ALife thermostat. I don't have any major problems that I have experienced with this setup other than the initial temperature setting.
Hope this helps.
D80