Definitely get the oil warm first, it helps a great deal.
I was a bit concerned it wouldn't get all the old oil out, so the first time I used mine I removed the drain plug after the Pela had done its work. I left the plug out while I did the rest of the engine service, during which time no more than a tablespoon of oil had drained out of the sump.
I suppose it all depends on getting the open end of the tube to the lowest point, but if you can it will definitely extract all the oil.