I have used PVC pipes in the past and all my horses have jumped them fine... However, they are too lightweight o be safe (unless your going to fill them with rocks like DiamondJumper has said)...
Case One: Jumping my old mare one day when a random but very small gust of wind blew the pole TOWARDS us while we were only 4 canter strides away.. Tell you what, my mare had never ever stopped and ran backwards so darn quickly.. Even after seeing dreaded evil pigs.
Case Two: Jumped my gelding over 1m, he had been clearing it but didn't quite clear it on this occasion and managed to catch it on his knees, it bounced up and got caught above his tucked up knees and then tangled itself in front of one front leg and behind the other, causing him to lose balance upon landing.. Lucky for us I had slipped my reins and he had his head to get his balance back, very very lucky, if he didn't manage to get his balance back and right himself we would have both ended up flat on the ground, I probably would have ended up underneath him. The only reaction he had was to take a couple galloping strides away from the pole when it finally hit the ground, while I regathered my reins.
If I had been using a wooden pole it would have just dropped, not bounced up in the first place.
Other than that, I use anything I can find, Tyres, buckets, 44gallon drums, old unused road cones, even an old plastic water trough type thing as a filler (very very scary according to Mitch haha!)
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms