Phil is correct - if you've got a lot of miles to cover in a day, there is no substitute for an 80 mph interstate.
Having said that, and done a bit of that last week in SD and WY, I have to say that the hours spent going 80 or 85 mph on a freeway are the most miserable time one (at least for me) can spend on a motorcycle. If you're going around trucks and cars, and waiting for that 65 mph truck to pass that 64 mph truck on that 80 mph freeway, it's just a PITA and it happens frequently. On my 4000+ mile trip to MN and back from NV earlier this month, I spent about 250 miles total on interstates, mostly on I-90 in 80 mph zones. The only places I was held up by traffic were on the freeways and in the Twin Cities metro area. Construction zones on the two lanes held me up a half-dozen times on the two-lanes, for a maximum of about 10 minutes - most were shorter. The mostly-deserted two-lane roads out in the middle of nowhere make passing the occasional truck or motorhome easy, most times you don't even need to slow down. And since most of the LEO's are patrolling the freeways, you can go at a reasonable rate and make good time between towns. The towns make a nice break, as well.
Even just a few days after Sturgis, most of the two-lane highways are a joy to ride. I had the pleasure of once again riding Hwy 16 from Buffalo to Tensleep WY, and a bunch of other mostly-deserted backroads. The experience between those and the interstates are a stark constrast.
Having lived for a lot of riding years where I had to ride a couple thousand miles to get to the "good riding," I learned after awhile that the good riding is everywhere. For instance, going across Nebraska can be a real chore on I-80, but take the two-lane roads farther south and you'll be treated to a lot of fun through the Nebraska Sand Hills.
I'll use the interstate occasionally as a connector road, or to finish a long day with a place as a goal, but in all truth I'd much rather spend the extra couple days and have fun riding rather than subject myself to a lot of interstate riding.
I guess it depends on your goals though, there are those that are "destination" riders, and those that go for the ride, with the destination being a secondary goal. I guess I'm the latter type. For instance, when coming home from Utah, I'd far rather travel an extra 50 or 60 miles running down to US50 to get home than going the shorter and faster way on I-80. IMO, 4 days of 600 miles on backroads are a lot more fun than 3 800 mile days on the slab.