Advantages
Less fan noise coming from the engine bay (no more noisy clutch fan!).
Less stress on the engine having to push a big clutch fan.
Possible improved fuel economy (I havn't noticed if its improved or not, my engine has been running really poor fuel wise (this started before the thermo fan conversion)).
Engine gets to operating temperature quicker (less engine wear).
More control over engine temperature - you can set what temperature the fans turn on
Improved cooling
And my favourite... The engine revs quite a bit easier and possibly more power? I don't know if more power has been proven but definately revs easier/quicker.

Disadvantages
More load on battery/alternator (although this hasn't caused any problems for me, just when the car is sitting still at traffic lights etc, the battery needle drops a bit).
Fan noise, The EL thermo fans can actually make a bit of noise when they're on, but still alot quieter than the clutch fan. I think aftermarket fans would be quieter but more work to mount properly