Extending jambs, Marvin Integrity window

How do others find the barbed plastic spline extrusion, offered as a feature of the all-Ultrex models of the Integrity windows, a good way to fix (meaning fasten) and align site-installed jamb extensions to the windows?
See the pic, attached here. The extensions shown in the pic are wood moldings available from Marvin, but you can mill your own from everyday 3/4 stock.
Edited 11/20/2008 1:46 pm ET by Gene_Davis