Frank, you've already gotten great responses about GrillGrates, and if Brian gives the thumbs up, that says a heckuva lot right there. For what it's worth, I'll add my $0.02...
I've been using GrillGrates on my gasser for over a year now, just sitting on top of my stainless grates. I absolutely LOVE them, and I'm not just saying that - they give me much better grill marks than I was getting with my regular grates, they seem to give me a larger "window" for when stuff is cooked perfect before it becomes overcooked, and you don't lose stuff down into the grill (e.g. asparagus). Like Mark pointed out, they even out the cooking surface to minimize unwanted hot/cold spots. I like them so much that I just ordered a set to replace my stainless grates directly (not sit on top). I'm told they will get hotter faster when used directly and not sitting on top of other grates. That increased efficiency means I'll need less gas for preheat AND for cooking (b/c the heat can be lower). I completely agree with others here, they're the real deal and worth the $$. -Mickey.