I have a friend who b/f with a shield for 11 months with no problems at all.
I used one with DS1, he wasn't attaching and we used it till I was able to wean him off when he was 10 weeks old, as the others have said I would start the feed with the shield on till after the let down and quickly whip it away and try to reattach without it, sometimes it worked and other times it didn't.
It was also suggested to me that when trying to wean them off it to not try at every feed, maybe every second feed until you don't need it all.
I also found that the feeds in the middle of the night were good times to try without the shield as he was sleepy and would just feed and go straight back to sleep. I would get in touch with a Lactation consultant so they can see how you are attaching etc.