Love this! I have a 2002 Subaru WRX. These cars are typically very hard to purge air from the cooling system due to a lack of radiator cap, and the cap being only on the reservoir. I sealed the cap for my type of radiator cap, and attached the funnel. I was able to run the car while adding coolant. This allowed air to escape, fluid to overflow into the massive funnel and causing minimal spilling.
The seal isn't perfect, so it does leak slightly when you use it, but much better than the old way for me at least. Previously I tried to purge the air out of the cooling system and it created a small volcano of coolant that spilled all over the place.
Additionally if you have poured too much coolant into the funnel, No Worries! You just use the seal to add the coolant back to the bottle. Like I said I loved using this product and has made doing a task that is usually a pain in the butt, much much better.