The best way to describe this song is simply "meh". It's fine for background music at a wedding dance, but other than that it sounds like it would be on some sappy daytime soap opera. Of course, when I do say it's "meh" I'm mostly referring to the writing here. The entire premise extends towards what you'd think it would - loving each other forever despite all the of eventual obstacles that the two will face such as growing waistlines and gray hair. There's a line about naming their children specific names depending on the gender, but aside from that it's nothing all that special or innovative.
Now, I would say Faith Hill sounds fine on the song in terms of her delivery and power (even if there's really no chance for her to show off that power), but Tim just sounds absolutely TERRIBLE. He's forced to go into his higher register for a good chunk of the song, and between that and him entering into his falsetto at points, he just sounds painfully uncomfortable.
Sonically, this is an area of the song I can respect. It's a piano driven ballad backed by acoustic and pedal steel textures, and really, it does do a good job of setting up the more serious atmosphere for committing to each other for life.
Again, this isn't a bad song, but it's just such a non-event single that I can't see doing much better at all than "Speak To A Girl". This is certainly better than that song, but so far the two have stumbled upon songs that feature some generic (or in the latter's case, clumsy) writing with decent enough (at best) other attributes. You'll hear it, you'll think something about it, and then you'll forget it in five minutes.
Listen: "The Rest Of Our Life"