I love 505 as well. When they had the .99 shipping I bought two cans but at this price I might have to get a couple more. I began using it because the weight of all of the pins was getting to be too much for my shoulders and wrists when I was quilting. I have also just started basting on my Cal King bed with no problems. I spray ( a little heavier on the edges) and let it sit for a day or so and then I start quilting. I've not yet had any problems with it not holding and my backing stays nice and smooth. I don't really see myself going back to using pins again.
Also, I have a Bernina 440 - so not a large harp at all but I've not yet had any problems with the spray not holding through all of the pushing and pulling that I do during quilting. I will be tackling a quilt that will be about twin size maybe a little larger in the next few months. I'm not anticipating any problems. I may spray just a little bit heavier on that one. I always give my quilts a pretty heavy duty washing when I'm done (much harsher than what I suggest that receipient do) so I'm not worried about all of the spray coming out.