Some of the best fisherman I know don't even use a probe. If you learn how to fish without one, you can do just as good as anyone with one. Speed and temp are important, but a proper spread is just as, or even more important. Thinking a fish will only hit at one depth, and one speed is ridiculous. I have watched fish rise 40-50' to come up to our riggers on the graph. The best investment you can make in this sport is trustworthy contacts. Get to know your fellow anglers, and share info.