I guess it depends on what you are comparing it too/starting from...
If you currently have a fixed fan, then as a viscous one is basically only active when the engine is hot, it allows the car to warm up quicker, and takes less power from the engine when the engine is cold, both of which help fuel consumption. Not sure I could ever say it had better cooling properties though, as it is basically the same fan, with the ability to slip when cold.
The best option is really an electric fan, which has no direct connection to the engine, and only comes on when hot. Better still, it can cool the engine, at low revs which neither fixed or viscous can, so is better when travelling at slow speeds.
Personally, if I was thinking of spending the money on any form of conversion, I would go electric, and stay away from the viscous.
