Ubuntu 14.10 64bits, intel i7, 4gb
Broken: 2.74rc2 & up-to-date master (22dfb50)
Short description of error
I don't actually know how this value should work, but I've tried several ways to setup the hook (vertex menu in edit mode, vertex groups). The only way it does anything, is setting the hook manually with a vertex group, but it just affect the vertex group as a whole (like a strength value)
Exact steps for others to reproduce the error
Just open the file. the radius value in the modifier doesn't affect the hook.