It gives you 10 rage instantly, then the 20 over 10 seconds.
As far as that talent goes in feral spec, I know that some tanks swear by it, while others think its a waste of talent points (compared to Primal Tenacity atleast). Personally, I've never noticed a difference tanking with or without Intensity.
Edited, Sep 26th 2008 2:17pm by Treehands