Bloc Loc Rulers are one of the best investments I have made in my sewing room.
The Bloc Locs have a groove on the back of the ruler that locks onto the seam to make the trimming up of blocks so much easier. Bloc Loc's eliminate slipping sliding and rocking and makes trimming my blocks 90% faster. If you want precise these rulers do that!
With some of the ruler sets there are cutting templates included as well as the trimming rulers.
There are also good tutorial videos available on how to use them and all the different blocks you can make with each ruler.
Bloc Locs are especially great when teamed with my other favourite sewing tool my Matilda's Own Rotating Cutting Mat.