I found mine really easy to fill with a regular garden hose, then in the spring to empty it I just drove up a hill opened the valve and it all drained out. That's the way they recommend to do it on the back of the bag.
Plus when it's water it is full of baffles so it doesn't slosh and then when it freezes you can still stick 500lbs worth of stuff on top of it. It leaves the bottom of the bed flat. So instead of intruding on bed space like a lot of alternatives it just raises the load floor up a little.