I tried a demo of the last one and found it somewhat frustrating; some of the random NPCs appeared to have superhuman visual acuity and managed to spot that I was a prospective homicidal maniac even though I was wearing a disguise and just standing around with all the other nobodies. I much preferred Mark of the Ninja's combined stealth/puzzle approach, so I might give this one a miss. That said, the homing briefcase is hysterical!
That's an intentional game mechanic. For every costume you wear, there's some people who know their co-workers well enough to know you don't belong, regardless of what you're wearing. If you see someone with a dot over their head, you have to avoid attracting any attention from them, meaning steer around them entirely, rush by, blend in, or, as you do, throw a suitcase at their head as they turn a corner.
I played the demo and had a lot of fun with it, though it IS very hardcore stealth and I don't know if I'm up to that for a whole game. I tend to lose patience in stealth games.