My parents installed and have used a fan similar to the the first one for about 20 years now for the air supply to their room, but it's a specific case. Their bedroom is on the top floor of their house and the HVAC run that feeds that room is very far from the air handler (on a horizontal plane), with the registers in the top floor of the house all coming out of the floor. Without the booster fan, no meaningful amount of cool air makes it into the room. So from that perspective, they work as intended.
I think the problem with the register fan is you're not necessarily making more air come out. The ducting has a finite capacity for airflow, and the fan shroud ironically limits what's able to come out of the register, almost as though the fan shroud is negating the extra negative pressure in the duct created by the fans. The only upside I can see to that one is that you'd perceive the air to be coming out colder, since it's coming out faster. But otherwise, it's just another way to distribute air around a room and at that point you're going to be much better off with an oscillating fan.
You're going to get more air from your HVAC by doing one or more of the following:
-Make sure all the accessible duct seams and joints are reasonably well sealed
-Insulate the ducting where you can
-Clean your air handler
-Clean your ducts
-Upgrade your air handler
That last one is the key - if your air handler was properly sized (and variable speed air handlers are far superior to and more efficient than single speed models) and your ducts were reasonably well sealed, you should be getting sufficient airflow from all the ducts in the house. But I suspect that yours is either underperforming because there is some constraint/restriction on the system, or because it is indeed not properly sized.
The only circumstance where these are actually going to help is in rooms where the registers are in the baseboard or in the floor and also a significant distance from the air handler.
Otherwise, I'd bet that you'll have much better luck by getting some inexpensive tower fans in the rooms where airflow is the issue.
ETA - The register fans definitely aren't something I'd recommend. I just poked through some of the reviews and they're designed not to boost airflow, but to actually help with managing the temperature in the room by choosing how much air to pull from the HVAC run while the system is on. The shrouds do indeed act as an airflow restriction, and the fans operate at variable speeds depending on the desired room temperature. Nothing that a tower or oscillating fan with a thermostat can't and doesn't already do.
