I got some at Badders and love them.I can leave them in the field and hoss cannot damage them.They are quite height restrictive but you can stack them safely.
i like them, height thing is a good point, im a bit of a whimp now and like the hight to go up gradually but poly block are one height or another.
also we got water in ours to weigh them down a bit to stop them blowing over or flying away if the horse knocks the jump, downside because they are so light!