Over the past few years, my husband and I have driven pretty much the entire length of I-15 (from the Canadian border to our home in San Diego County). There are some really beautiful stretches of road, as far as scenery is concerned, and as of last summer, in excellent condition. There were a few areas of construction, as there always will be on interstate highways during spring, summer, and fall. (There are two seasons on the freeways: winter, and construction.) They may give you no trouble at all, you may be down to a lane in your direction, or you may have to slow down a lot.
Our favorite sections of I-15: Virgin River Gorge, which is located in the NW corner of Arizona just south of St George, UT; climbing into the hills north of St George UT; Pocatello ID to Idaho Falls, ID; Butte MT to Great Falls, MT.
I can't speak for the border crossing at Sweet Grass, MT recently. However, we did cross the border at Chief Mountain (just west of there) last summer, going north, and then into ID at Eastport going south, three days later. Everyone's experience is different, but the wait was 30 minutes northbound and would have been the same, southbound, had they not opened another lane just as we pulled up. Recommendation would be to have an inventory of what you are bringing, such as medications, electronics; make sure your medications have a prescription on them. The BP agent started to give us a hard time trying to return to the US, saying that they assume we bought everything in Canada (all we bought were some t-shirts!), until I offered our trip inventory list. Then he changed his line of questioning.
I will add one thing about the fast food thing, to what AZBuck suggested. Up in Canada, if you find an A&W, realize one thing: they still serve root beer floats up there in real glass mugs, just like they USED to do down here in the US but no longer do. That was one US chain that I didn't mind doing, in Alberta!
Donna