I also have had good experiences with Great Lakes Scrip. We had a huge issue last holiday season with a lost gift-card (my fault) and they bent over backwards to help us. In regards to backorders, if you look at their notices regularly and check their turn-around calendar, they are pretty honest about when you wil receive your product. Make sure that your scrip coordinator has email updates from GL Scrip or that they check the "What's New" section of the GLScrip website frequently to get a better knowlegde of turnaround times. Good luck.
I have had no problems with Great Lakes Scrip. We switched to them from Manna Group last year and I have found them to be much better. Some of the %back are not as high, but their shipping is much lower.