Do you think they are better than a cast iron griddle or skillet?
Are you askin' me or devo?
You wanna have the right tool for the job...some stuff does better in cast iron. I would melt cheese in CI...I PREFER to cook salmon on the sheet because of the extreme non-stickiness of it all - oh, and it cooks real well thereon.
The sheets are simply one more tool (kinda like frog mats) in the toolkit. They are not the "one-and-only", but they are very useful in specific applications. They aren't as hard to pre-heat and cool down as CI. I haven't even TRIED to sear anything on them cause they aren't really made for that. They DO hold up when used with my Quantum IR, so they are pretty heat resistant - but I haven't tried to see where the max point is.
No - I wouldn't try to cook yakisoba or hash browns on grill sheets, but salmon, shrimp, onions, and veggies, etc do very well.
Maybe you weren't askin' me about the CI...in that case I defer to my esteemed colleague - The Devo!