If you're getting full voltage out of your factory 120 amp alternator, no need to change it IMHO. However if you need a rebuild, it's a good , cost effective set up. Bought mine from Advanced Auto for under $130.
Yup, it's a bolt on that fixed most of my voltage problems I was experiencing with an old tired alternator (the other half of my problems turned out to be grounding issues).
Save your money to put to a better use. The factory alternator is more than adaquate unless your aftermarket electronics are numerous and you operate everything at once all the time.