When it says "...you can use your ranks in Sense Motive in place of your total Will saving throw modifier", are you using just the ranks in Sense Motive or does it include anything else?
Just the ranks, which is generally quite good. At 20th level, for instance, you'll get a +20 that way (assuming you do the obvious thing and max out Sense Motive), whereas from class & Wisdom you'll get +6 to +20 depending on the class and how much you've invested in Wisdom. At the upper end of that range there's no help, at the lower end you've effectively got a +14 bonus to your Will save! It's less impressive at lower levels, but still good, esp for classes with a poor Will save progression and/or characters who aren't Wisdom-focused.