Benefit: Pick a class when you gain this trait—your caster level in that class gains a +2 trait bonus as long as this bonus doesn’t increase your caster level higher than your current Hit Dice.
So, this means that if I have a 3rd level Fighter/3rd level Wizard who has this feat, the character's number of spells per day is as a 3rd level Wzard's, but a Bull's Strength spell would last for 5 minutes instead of 3, a Shocking Grasp spell deals 5d6 damage instead of 3d6 and saving throws against his spells are at +2 DC, correct?
Correct.
all except the saving throws is correct. since thats based on your characters stat and not his/her caster level.
Missed that. Good catch.
...I don't know if this is relevant - but the trait is banned in Pathfinder Society Organized Play (as far as I remember - to be certain check the additional ressources page).
Sounds like a fun trait for rangers.
I've used it for both ranger and paladin. A must imo.