I first encounted Bloc Loc rulers at a quilt show where they were being demonstrated. Bloc Loc rulers have a channel that snugs up to the seam, eliminating slipping and ruler rock when you trim. I've found that the best way for me to get accurate flying geese blocks is to make them a little bit over-sized then trim them to size before sewing them into larger units. There are a number of different rulers out there that help you do this; I have and use both the Fit To Be Geese ruler from Open Gate Quilts and Deb Tucker's Wing Clipper, but my new favorite is the Bloc Loc.
See how the channel in the lucite ruler snugs up to your seam line? It is so easy to cut around this ruler accurately.
The only drawback is that each Bloc Loc Flying Geese ruler makes only one size block, unlike the other rulers which work for a range of sizes. So I've been buying the rulers as I have a project come up that warrants its use. My latest is the 3" by 6" size. I'm using it to construct the 2013 school auction quilt. Oridinarily, I prefer smaller blocks but I find that a bold, graphic look works best when using this high contrast color combination (gold and blue are the school colors). There will be a lot of flying geese in this quilt and this ruler is helping me construct them efficiently.