I have the same heater. I don't like it very much, but I use it sometimes. I have a 30 (maybe 35 gallon?) rubbermaid trough that I use in the winter. Luckily our temps don't get into freezing too often. I turn it on in the morning when I get out of bed, and by the time I leave for work (about an hour later) I unplug it. It thaws it out nicely.
The one I prefer is this: http://www.callisters.com/assets/ima...s/15dp_api.jpg
Again, I only have to plug in at night, and it's been a week since freezing temps, but I like that one, because the horses cannot move it, and it's fixed, so the risk of it melting my trough (100 gallon) is next to impossible.