Scent blocking clothing, I haven't seen a big difference in.
Scent killing sprays though, are FAR from a gimmick. If you use them right, they make a huge difference. Most hunters just plain don't use them right, you have to spray down thoroughly, and not only get your clothing, but also your hair which is what distributes your scent the most. I rub it into my hair and beard on every hunt, and spray down my socks before putting my boots on. The difference in what you can get away with scent wise, is remarkable.
And like others said, you can never fool a deer's nose completely...........but you can make them think you are much further away than where you are, or that you were there a long time ago and not right now. I've shot a P&Y whitetail almost every season for near a decade, fair chase and pretty much public ground, and I swear by the scent killer 99% product.