Remove shoes, remove belt. Walk through metal detector. Forgot to remove change, so metal detector beeps. Security guy asks if he can scan me with hand-held scanner and I consent. He sweeps it all over, oddly it beeps in places other than my pocket. But it also beeps at my pocket. Security guy asks if he can pat those areas down. I consent. Turn around. More waving, more beeping, maybe more patting down.
Happens to me half the time I walk through a metal detector. Recently, it even beeped when I removed everything I could think of - I don't know what caused the big and small metal detectors to go off. After a simple pat-down, the security guy was satisfied.
If a scanner can even eliminate the time spent doing that, then I'm happy.
Millimeter wave detects objects, it does not generally provide enough context to identify those objects with any certainty. So any object not trivially identifiable in silhouette will still get you pulled aside.
As it detects any object, as opposed to just metal ones, it dramatically increases the chance that you, or any other passenger in queue, will forget something the scan can't rule out and create further delay.
The scan itself is far slower than the metal detector and slower than even the bomb-sniffing machines. Its presence will increase your security queue time, even if you somehow become less forgetful and have hassle-free experiences with the new scanner itself.
You'll have traded a bit more of your privacy and liberty in exchange for increased delay, increased aggravation, increased travel costs, escalated privacy risks and it still won't make your plane trip any safer.
