We've already been using on the bestselling pet fountains on the market: Drinkwell Pet Fountain (range from about $30 to $46 for the deluxe).
The Fountains are have a large footprint and have to be strategically placed (lots of floor space, and near an outlet). This may be difficult for smaller kitchens. Plus even though they are much larger water capacity (ranging from 50 ounces to over a gallon), you still have to check it daily. Your pet can spill it, there's a chance of leaks, the motor can get clogged with debris, & if it gets empty you can burn out the motor.
Additionally, the Fountains require larger filters that also need frequent replacement (and they're more expensive).
THE HY-DRATE: BEFORE USE...
The complaints about charcoal powder residue are a non-issue. You get the same charcoal powder in home water pitcher filters & home water filtration for human consumption. It's perfectly harmless, but you should read the instructions & rinse off for about 10-20 seconds before use. That goes for human water filters as well - anything with activated charcoal.
As for complaints about the motor safety ...the submersible pump used here is the same type and even same brand used in dozens (maybe hundred) of other products (discussion below). I'm not saying it's foolproof, but rather, if you're going to use a product with a pump such as a Filter or Fountain, this product is no more (and no less) safe than any other product.
THE HY-DRATE: TESTING...
This is a clever little device, almost nothing to it. Easy to take apart & rinse out. Easy to adjust flow rate.
The Hy-drate uses a normal little pump (like a fish tank pump), appears to be the same brand as they use in the Fountain. I find the claim that it's "whisper quiet" a little disingenuous - same noise level as the Fountain, as a fish tank pump, as the pump in our little tabletop zen garden. Not loud, but not any quieter than any other pump.
They claim the device fits in any pet bowl, BUT, the device is bulkier than it needs to be and requires a 3.5" diameter bowl with a FLAT bottom (look at pic - you'll notice they gave the cat a HUGE bowl like 14" diameter). Our pet bowls are not flat like that, so we had to dig out a large soup bowl.
It performs just fine. I think it does encourage my cats to drink more, which is a particular perk if your pet has ever had a urinary infection.
THE HY-DRATE: CLEANING...
PLEASE note, you still have to wash out your water bowl, and change out the water, on a regular basis. One user reported that this product "caused" a slime build-up. This is just a pump + filter. There are no chemicals here that "cause" slime. What he's describing is a build-up of saliva (or, if like my cats who clean themselves in the water, possibly a buildup of dirt & hair, too).
There's nothing we can do to stop our little guys from playing in their water bowl :) ... which is why we like the idea of a Fountain or Hy-Drate filter. But please realize, a little 3-inch pump isn't going to "clean" and disinfect the water, the bowl, and itself. We wash our bowl once or twice weekly (dishwasher/sterilize), which seems to do the trick.
MONEY & TIME SAVING TIPS...
(1) Make sure never to run it dry - it will kill the motor faster than anything.
(2) Turn off at night (alternatively, only turn it on at night, whatever is best for your schedule). It doesn't need to be on all the time to work. This will almost double your filter life, save electricity, save motor life, dramatically reduce the chance of motor or adapter overheating, and your pet still gets all the benefits of filtered water.
(3) Filter Replacement - These filters are super-tiny, need replacing very frequently if you run it 24/7.
However, I took the unit to the pet store and there are lots of activated charcoal filters that fit just fine in this unit - that both last longer, & are less expensive. I know I'm cutting into the company's profit model by suggesting this. Also, ask for help at the store to make sure the replacement filter doesn't block water flow or disrupt your device's normal operation.
DRAWBACKS OF CHARCOAL FILTER...
This is just a charcoal filter. In my book, it's better than nothing, but I'm not sure I consider it "filtered" water (i.e., not distilled, not reverse osmosis, etc).
Charcoal filters will trap and remove chlorine, water sediment, and dirt/cat hair that your pet tracks to the water bowl. This is all very good.
It will NOT filter most toxins in water (sodium nitrates, etc). One recent study found traces of 80,000 chemicals in city water, and this was in one of the top rated cities! (traces of drugs like Prozac, herbicides like Atrozine, etc). For this reason, many users report putting filtered water in the bowl to start with.
NEGATIVE HEALTH EFFECTS OF FILTERED WATER???
Ok, if you restock your Hy-Drate or Fountain with filtered water (like distilled, or through a good tap water filter), this is great for removing toxins, but may also deprive your pet of essential trace minerals found in the water supply. I AM NOT A VET, however, I know that ALL mammals (yes, especially humans) require trace minerals.
So we supplement their food once a week or so with a small dose of something like this: (Barefoot Coral Calcium) - which has 80+ essential minerals. Our vet has given us his gold stamp of approval. Our dog is 13, our cats are 14, and they are happy & healthy as horses. Check with your vet first - don't take my word for it, especially if your pet has special medical needs.
Here are our other recommended practices for most pets: In addition to filtered water, we give our pets a daily (Vitamin & Mineral Spray) on their food, a healthy dose of fresh/raw food (like carrots) to make sure they get some enzymes, fish oil supplements for brain health & coat/skin health, and other supplements as needed.
Hope this helps! Good luck!