I rode a long stretch of US 15 years ago and it was good then. I'd assume it would still be pretty good as, like with US 301 in NC, I-95 takes most of the N/S traffic through that part of SC.
I'm not much of a fan of US 17 in SC... My son and I rode from Yamassee, SC to Charleston two months ago and took the section of 17 you are considering. It was not a very good ride at times... there was a section of road work 14 miles long. Both directions of travel ended up on what would normally be the southbound lanes. When we got to the rail trail going into Charleston the trail was closed due to construction. The section of 17 going into Charleston was a miserable piece of road. As I recall it was typically 6-7 lanes wide with curbs, no shoulder and a lot of traffic. http://www.crazyguyonabike.com/doc/page/?o=1&page_id=336998&v=7K
Perhaps the road work is done, and the rail trail should be reopened by now based on the signage, so perhaps it would be OK now.
One issue in that part of the SC is getting across the Edisto River. There are not many crossings and if you are heading to the coast from Walterboro 17 is pretty much the option....
Another stretch of 17 heading into Savannah was a bit better and that route is used by the East Coast Greenway as the preferred route. http://www.crazyguyonabike.com/doc/page/?o=1&page_id=336613&v=8D
Phyl and her husband Neil, a couple we met in Williamsburg, VA who were southbound, didn't care much for the stretch of 17 they rode in northern SC either. They did however take it into Myrtle Beach, SC. http://www.crazyguyonabike.com/doc/page/?o=1&page_id=341196&v=5zÂ