GrillGrates will definitely reduce flare-ups and even out your temperatures. I use them on my daily driver Genesis Platinum and before that the same set on a Silver C. They are a regular at SCA steak competitions, because they faithfully deliver razor sharp grill marks:
Compare fish on GrillGrates (top) with very nice cast iron piece (bottom)
Another advantage of GrillGrates is that much of the grease, etc. gets incinerated at the grate level. You do have to clean them from time to time, but your firebox will stay a lot cleaner and you will not get nearly so much grease inside.
Are their disadvantages?
Well, for one thing, although GrillGrates have plenty of holes, they still hold block a lot of heat down below. You need to adjust your settings to something more like 3/4 power or risk trapping too much heat. The grates will be searing hot, even so, as you can see.
Some people feel that using GrillGrates alters your grilling experience to the point that you are no longer "grilling" but are more like using a stove top. I haven't found that to be my own reaction. You can put a smoker box underneath (or above like in the picture), and even without one there is still sizzling smoke. Seldom any real flare-ups, though.
The most controversial aspect of GrillGrates is that they are made of anodized aluminum. The manufacturer (of course) claims there is absolutely no risk from using aluminum in this way. It is at least a reasonable argument that in a hot grilling environment, with food acids and scraping to clean your grates, that the anodized coating is likely to come off in places. That leaves you, then, with the issue of whether or not cooking directly on aluminum can cause disorders such as Alzheimer's. There are those that feel strongly that aluminum is a direct cause and can point to results they have seen. Others, including the National Alzheimer's Assoc, say that there is no causal link. It has also been pointed out that there is a big difference in simmering something with high acid like tomato sauce in an aluminum pot for hours vs. five or ten minutes of food just sitting on GrillGrates. This is a highly personal decision that each person needs to make and be satisfied with. If you have doubts, don't use them.
For me, GrillGrates deliver reliable results and are much more forgiving than any other grate material I have used. I have not been persuaded, at this point, that they pose a health threat to me or my family.
I have done side by side cooking with GrillGrates and thick stainless rods and while sometimes the results were close, the stainless rods never did better and sometimes didn't do nearly as well as GrillGrates:
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