A few years ago I used the Equitrol feed-through in addition to removing manure and pee spots daily. The next year, I had trouble finding it and about half way through the summer I realized that the flies weren't any worse with out it. I had not heard about the colic problems associated with it, but now I'm glad I stopped using it. Another thing about the feed through is that the chemicals in it only stay active for about a week. After that, any eggs which get laid in it can hatch. So if your manure isn't removed from the property weekly, it would be a waste of money.
I've heard good things about the preditors but I personally wouldn't bother with them. It's so windy at my place that they would just blow away!
Nice thing about wind, is the flies get blown away, too!
I like Tri Tec and the Endure rollon. They seem to work the best for me.