I've had a NV Silencer on a GF4200 for over a year now and no problems. The fan is as quiet as when I first installed it.
However, I would mention that when I pulled the card out of my system recently, the edge of the heatsink closest to the fan was almost completely blocked with dust. To clean out this dust I had to completely remove the Silencer from the card, remove the screws holding the duct work and use a vacuum cleaner to suck up the dust. Other than that, I've had no problems with the unit.
EDIT: I also have an ATI Silencer on a 9700Pro that's been used occasionally for over a year. It works so well in my HTPC that I forget that it's in there!
Just remember, these coolers are LARGE and are a tight fit even in large cases.