At PAX, I seem to remember putting on a disguise and walking at a distance past a guy. He spotted me instantly. Maybe that was a non-final build, but I remember trying a few times and finding that it wasn't that fun. But they also didn't have any info there for you to show you how to play, so I didn't even use the new instinct mode or whatever.
Hitman Absolution tries to be a little more realistic regarding the way disguises are handled, compared to the previous games. If you dress up in security guard clothing, and an enemy security guard looks at you, he won't recognize you as one of his peers and he'll get suspicious almost instantly.
Not so much for other types of enemies in a "different group" than the ones you're dressed up as. They won't get you in trouble as quickly, but if their eyes linger on you for too long, they'll still get suspicious.
The game puts more emphasis on hiding, using cover, and making distractions/redirections. You can't just put on a costume anymore and instantly be able to walk freely wherever you want. If you do decide to walk around in plain sight, you still have to be extremely tactful, and you have to make sure the wrong people don't lay eyes on you.
And yes, using "instinct" is critical when disguised. Sometimes you have to use it as a last resort when enemies are about to spot you. When done correctly, Agent 47 will casually put his hand over his face, pull his hat down, or brush his forehead so people can't get a good look at him. It's a neat mechanic IMO and it requires a different playstyle than the older titles in the series.