I did a 300 mile trip a couple weekends back, open road, some steep hills and it did a great job keeping the set speed. Normally I don't use it because most of my driving involves NYC/Long island traffic jams where to me it's kind of a useless feature without it being adaptive.
That's what I do with mine and haven't needed to look at or touch my phone at all, just wish apple supported waze/google maps, Apple Maps still needs a lot of work especially if you live in a city and need traffic info.