It is pressurized, and it relies on the water pressure of your fill faucet. So I would think that soon after you start spraying, the pressure is going to equalize. Water can't compress, so the only way to build up pressure is in expansion of the hose or expansion of the container. If the container contains an expanding bladder, that would give you a bit longer pressure, but I think it would drop rapidly.
Either way, I made up a throwaway email address and signed up to win a free one. I'll let you know how it works. ;)
[EDIT] After reading reviews on Amazon, it does contain a bladder to maintain pressure. Seems to get mostly good reviews.
I've looked into this before but didn't want to spend the money. I ended up using a 2 gallon sprayer from Home Depot and attached a sink sprayer to the end of the hose. All in all. 2 gallons of pressurized water for less than $30.