I have had 3 of these over the years.

Pros:
Large enough supply of water for my two big cats

Circulates the water supply

Slight Filter effect

The cats like it...

Cons:
The water gets slimy. I clean all the parts about bi-weekly and use filtered water from my Brita filter - seem to stay clean longer....

The pump seems to wear out/ wear down and can be noisy. I also had one on which the cord wore away right where it feeds into the pump. I was concerned about possible electrocution so I tossed that one out!

Pros: Large supply of circulating water

Cons: Gets Slimy, Wears out

The idea of the fountain does work. The cats do seem to drink more water! However, I had more than one and it was difficult to keep the water clean with or without the filter. I thought it might be something in my water, so I used the Britta filter first and then poured it into the fountain. I still had the same issue with black slimmy mold. I would have to take apart the pump to get the slime out as well. Plus the round edges are hard to get into to clean as well. I tried an old toothbrush, but I still couldn't reach all the needed spots.

Pros: Cats like it and drink more water

Cons: Hard to keep clean