I've ridden Route 17 east to west from Corning to its end at Barcelona. The biggest hill going that way is Morton Summit west of Wellsville, about 3/4 mile of 6% (more or less) grade. There is a long upgrade from Salamanca to Little Valley of about 2%. If you're going west to east on that stretch then by far your worst climb will be out of the Lake Erie valley at Westfield. I'd guess 10%.
Biggest drawback is that the road is busy. You have full 8' pulloff lane for a bike route for most of it, but I found after three days of riding that the endless traffic noise really started to bother me. There is only one not-for-the-faint-of-heart stretch of about 2-3 miles in Celeron west of Jamestown -- 2 lanes each way, a concrete curb and nowhere to go with cars passing about 2' away.
For fun, try to get Chatauqua on Sunday when they are in season and you can get in free. Also consider detouring through Hornell to ride the Canisteo river valley. That's a sweet ride.
All in all, it's a well planned route.