Seems like many people are asking this question so I will provide some details.
The short answer is the Ohio to Erie Trail is paved in the southern end and crushed limestone in the north.
Here is more information…
The Ohio to Erie Trail spans the state of Ohio from Cincinnati to Cleveland following abandoned railroad tracks and canals that have been converted to paths for bicycles. The Ohio to Erie Trail utilizes existing trails and the organization is working to close the gaps to achieve the dream of a cross state off road route for non-motorized transportation. My estimate is the route is 60% asphalt paved trail, 25% crushed limestone trail, and 15% road route.
Traveling south from Cleveland the Ohio to Erie Trail currently begins on the Ohio and Erie Canal Towpath Trail at The Cleveland Metroparks Ohio and Erie Canal Reservation. This section (approx 8 miles) is paved and fabulous and fun with lots of twists and turns. The trail switches to crushed limestone at Rockside Road as the trail enters the Cuyahoga Valley National Park. The crushed limestone trail surface is not as bad as it sounds. The trail is smooth and worn in as it is nearly 200 years old. You can still travel at a decent speed and the scenery is spectacular. The downside is that it is dusty and it will get everywhere. You will need to clean your chain and wash your face!
As the Cuyahoga Valley National Park ends the towpath continues into Summit county and Akron. There are some sections that are paved and some that are limestone. It switches back and forth with pavement near bridge crossings and trail heads and crushed limestone in more rural sections. There is a wooden boardwalk on the southern end of Akron and some newly paved sections that eliminate the old on-road route through Akron. The Ohio and Erie Canal Towpath Trail is now contiguous from Cleveland to Massillon (the Towpath continues south to Zoar in Tuscarawas County but Guy and I cannot verify the current condition. We will ride it soon!). At Massillon you pick up the Sippo Valley Trail which is partially paved with asphalt and partially crushed limestone. There is about 1.5 miles of trail that is large gravel and is very uncomfortable, but it is short.
At the west end of the Sippo Valley Trail you arrive in the tiny town of Dalton where you pick up a road route to get to Fredericksburg. It is rolling hills with a few steep ones and is very scenic. In Fredericksburg you pick up the Holmes County Trail which is paved, one side for bicycles and pedestrians and one side for horse and buggy. The Holmes County Trail ends at Killbuck where you pick up a road route to get to Brinkhaven. Again, rolling hills with a few steep ones and a section of road that is on US Route 62 which is 55 MPH and has big trucks. Its challenging but doable.
In Brinkhaven you come to The Bridge of Dreams that crosses the Mohican River and connects to the Mohican Valley Trail which is unpaved, bumpy and full of ruts below a surface of fine baby powder like dust. It is approx 5 miles long. I do not recommend it. Its a tough choice because the alternative is Route 62. We used the Mohican Valley Trail going south and Route 62 going north.
On the other side of Mohican Valley Trail is Danville where you pick up the Kokosing Gap Trail which is paved and perfect. The rest of the way to Cincinnati is all paved. Some sections are on the road but the crushed limestone is behind you. Some sections of the Little Miami Scenic Trail near Cincinnati are paved but getting pretty bumpy as the trail cracks with age. The sections going through London and Madison County are perfectly paved and there are sections where the asphalt has a seal coat which is unbelievably smooth.
Please do not let the crushed limestone surfaces discourage you from trying this trail. The biggest obstacle is the friction and dust. Just slow down, loosen your grip, and enjoy the scenery.
If you want to eliminate some of the crushed limestone riding consider using the Bike and Hike Trail which is paved and runs on the ridge top above the Ohio and Erie Canal Towpath roughly between Valleyview and Stow. (The Bike and Hike Trail continues south but Stow is good place to get down into the Cuyahoga River Valley to pick up the Towpath.)
Here are a few links to photos of the trail and other descriptions of conditions and the daily postings from when we rode Ohio to Erie Trail north to south and back in May and June 2012.

Ohio to Erie Trail Description Posted on May 29, 2012 by Carolyn

Southbound day 1, use the arrows to continue reading daily entries
Stow Ohio Posted on May 19, 2012 by Guy

Northbound day 11, use the arrows to continue reading daily entries
Frontier Campground, Silver Springs Posted on May 30, 2012 by Guy

Ohio to Erie Trail: Holmes County Trail to Mohican Valley Trail: Killbuck to Brinkhaven Map

Information on the Bike and Hike Trail:
day ride
overnight ride