The biggest upside for me of the electric fan is that it doesn't run all that often, and doesn't spin at all when I first start the truck and it's wicked cold in the winter. Helps it warm up a good bit faster to have no air moving around the engine bay. It's also noticeably smoother with the fan off, but I don't notice a huge efficiency difference from driving around with the fan on vs. off.
If you do a lot of highway driving, the electric fan will almost never turn itself on, which is good for efficiency. On the trail though, it turns on and off so much that I often just switch it on to avoid wearing out the fan relay with constant cycling. Similar story driving through town. I have a pretty weak alternator (the Toyota alternators with vacuum pumps on them are just not strong) so I wish I could get a mechanical fan in there for that reason alone. A 50A alternator doesn't leave a whole lot when the fan draws 25A.
Marc: I suspect that a mechanical clutch is a more efficient way to turn a fan than via alternator, but that the fact that an electric fan can be off so much more of the time is what bumps them up in efficiency...
Dan