Lucky you - my hatch opens very slowly and not very high, to such an extent that I have hit my head on it a few times when any other hatch would have opened but the Prius is still opening very slowly and to a ridiculously low height. Of course if its raining the experience is made much more unpleasant because the shopping will be drenched with the run off from the hatch which seems to be intentionally directed INTO the trunk - what a design disaster this car is. Toyota must have engaged some trainee designers to build this car and ignored cars they had designed previously and all the other cars designed by any other manufacturer
If your car is six years old, probably the hatch struts are wearing out, which would explain the symptoms you describe. They aren't difficult to replace, but a little expensive. The problem you describe is worse when the weather is cold than when it's warm.
Get over it. I tend to look at the car, test drive it or even hire one for a few days before buying one. Go buy a Holden or ford and enjoy breaking down and using lots of fuel. Simon
