I have a spidot on my plastic primary. I love it. I works fine...never a problem.
I usually drain 90% of the beer out...then put something stable underneath the primary to tilt it and get the remaining few percent out....just be careful not to create waves that will disturb the seditment. The hole on mine is about 1.25 -1.5inches from the floor of the bucket....any lower and you will get sediment when you open the spidgot.
It also is great for bottling: just cut a 6- 8" piece of hose, attach with a stainless clamp and you get quick, clean and easy way to fill your bottles.
One more tip....when cleaning and sanitizing, open the spidgot to leave the sanitizer flow through so all is sterilized.