It doesn't work well with poly blends at all. I've got a Roebucks coverall (got it from Sears) and it's a 65% Polyester, 35% Cotton blend. It was originally a Navy blue, but I was planning to lighten it up to make a Jango jumpsuit. So far, bleaching hasn't worked too well, and the Rit color remover has also been ineffective. I first tried half a pack (to see how well it would work) and there was almost no difference. I then used the rest of that pack and an additional full pack, and tried again. There was still very little difference afterwards.
Then, I got serious. I tried using three packs at the same time. Once again, there was very little change. At this rate, it'll take 50 more to get it to the right color... maybe. :angry
When I get home, I'll post a picture of the new color compared to the original color.