We used to use Balsamiq for mockups, however Expression Blend 4 and Sketchflow has some compelling features.
Sketchflow is missing a sketch style grid among other things, so we looked to Sketchables to provide.
Referencing Sketchables.Silverlight.dll wasn’t enough, the new controls weren’t appearing in the “Assets” panel. Initially I tried referencing the designer files, but that yielded no results. Reading the blog comments from hardcodet.net there was the following one-liner:
The libraries in the “design” folder are automatically picked up. They are used in Blend in order to provide the design time support.
The “Design” folder must sit below the folder which contains the referenced Sketchables.Silverlight.dll; Expression Blend 4 silently scans dlls in the design folder and loads them into the Assets panel.