I hesitate to post on this subject, because it is indeed contentious. I am in attics everyday. I walk all over them during a typical home inspection. Here is a good objective article on the subject;
Don't Let Your Attic Suck | The ASHI Reporter | Inspection News & Views from the American Society of Home Inspectors
Make sure you also click on the attached articles within this article above.
Part of the problem here is that I think there is a bit of the "Placebo Effect". Once a person spends hundreds of hard earned dollars on something they will lean towards feeling that it was a good expense. Added to this is the fact that in some cases, the attics ARE a bit cooler. SO....the question is, what are you trying to do here?
Usually the answer is, "Trying to cool the attic so my A/C will run less". The problem here is that air is going to be drawn from it's least restrictive space, which appears to me to be the saucer and ridge vents. This can cause "short cycling" and then stratification of the air below. The air then just above the insulation will not be that much cooler. In short, ACTIVE attic ventilation can disturb the natural passive ventilation the attic is designed for.
Personally, if I were to spend my money I would first consider adding a bit more insulation first, as even the newer homes are currently an R-30. This meets code of course, but insulation is so inexpensive that going to an R-40 or more would have a payback, even with the law of diminishing return.
Many people get confused between having a fan on the roof or gable to exhaust the attic, and having a fan to exhaust the air out of your garage INTO the attic. They are completely different systems. If your goal is to cool down the garage by pushing air into the attic and out the ridge vents...this is different, and could work well as long as you have sufficient air INTAKE to make this work. Understand that I am not addressing any fire or wind mitigation concerns that this may impact. I am also not addressing how this affects the normal attic air flow from your soffit intakes to your ridge vent exhaust.
Lastly, please know that I have no "horse in this race" and do not care if you agree with this or not, or if you buy one or not. I do not sell these or benefit if you have one or not. I do NOT want to step on the toes of those who feel different than I do on this subject, or make a living selling these. Folks seem to have strong feelings about this, and I do NOT want to make enemies here....I only want to help by offering some articles written by experts smarter than I am.
Folks ask me questions everyday, like what is my opinion on Gutters, Water systems, Roof vents, Solar tubes, painted driveways, Lanai drainage, etc....My opinion is just that. I try to be knowledgeable and unbiased and try to do good research. Your opinion may be different, and I am not always right...(ask my wife)....my only goal is to help.
Respectfully, Frank