Recently, two from Cherry Point MCAS were making T&G's at the local airstrip. Living right under the downwind leg of the prominently active strip offers plenty of eye time for viewing CPC visitors.
I can say, they are remarkable AC to watch, but there's no way in hades the thing will ever "sneak up" on a living enemy.
they claim they are quieter than a Cobra or Chinook. Obviously its not an Apache.
http://www.globalsecurity.org/military/systems/aircraft/v-22-survive.htm
Compared to both helicopters and conventional turboprops, the Osprey has a lower acoustic signature due to the tiltrotor's reduced rotor rotational speed. It also uses very low thrust for cruise propulsion. The V-22 flying in aircraft mode produces a distinctive sound, described by observers as a "throaty and muted hum - more like a vehicle than a helicopter." The observers noted that, in combined operations, the steady buzz of the MV-22 was frequently masked until the last minute by the "whop-whop noise" of AH-1 Cobras and UH-1 Hueys that were supporting nearby. Overall, as compared to the CH-46, the MV-22 is less noisy while in the aircraft mode, and provides comparable acoustic acquisition cues while operating in the helicopter mode.
The acoustic siganture of the Osprey is about 75% less than that of the CH-46E and CH-53D that it will be replacing. That along with a 95% reduction in its IR signature, its greater speed and different ingress tactics mean that there will indeed be plenty of unfriendlies that will be surprised to see it.