Well, I bought two of the Tunze ones that are rated for ~55 gal each, and put them in my 100 gallon Long tank on opposite sides. I simultaneously unplugged the 4 powerheads I had been running to supplement my overflow return.
The water circulation is much better.
However, I think the best part is that the tank runs significantly cooler. Before, I had to keep it uncovered to keep the temperature at 83F. Now I can run the tank fully covered (thus slowing evaporation) and it runs at 78F. These things are FAR more efficient. I am thinking this may make my electric bill marginally better, but the advantage in terms of temperature control is phenomenal.
I also learned the warnings about the magnets being dangerous are not over-stating the case. I got the skin between my thumb and fingers caught between two magnets as they snapped together, and they are almost impossible to pry apart even if you have the use of both hands.