A Ford dealer will tell you that the belt will last 100,000 miles. If you ask me, I'll say it is an on-condition replacement. It is all up to you. If you don't like how it looks or you question its integrity, change it.
It takes me about 20 minutes to change a belt on a v6 tour. Not as big of a deal as some make it out to be. The hardest parts are releasing the tensioner, moving the tensioner to get the new belt on and routing the belt over all the pulleys. Other than that its not bad.
The tensioner, as long as it keeps the belt tight I'd say its good. But, when you take the belt off, turn the pulley on the tensioner, and grab onto it and see if you can feel any play in its bearing. If it sounds gritty as you turn it, feels gritty as you turn it, or if the pulley bearings have any play, replace the tesioner.
Remember, you want the tensioner to last as long as the new belt so you don't have to take it all appart again a year or 2 later when the tensioner quits.
my .02
