I biked this trail on the last Sunday in April 2012. Not sure if it was meant originally to be primarily a horse trail, but that certainly seems to be the case now. I biked the 20 mile round trip and encountered many horses/riders, but made it nearly all the way without meeting a single other biker. Finally there was a father riding with his young daughter, out for probably just a brief ride. So if you have biked other rail-trails and expect to encounter numerous bikers like you have on those trails, you should not expect that here. You might be the only one.
As further evidence that bikers are scarce on the trail, most of the horses I passed were skittish of me. After the first time, I rode as slowly as possible past horses, so as not to alarm them. I mentioned to one lady that the horses didn't seem accustomed to bikes. She said, "They're getting better. But we're happy to have you here."
* Even though it's mainly an equestrian trail, you definitely CAN ride the entire length on a bike quite comfortably. And you should. Let's get more bikers out there! :)
* As you might expect, there's 'horse hockey' (as Col. Potter on M*A*S*H used to say) and plenty of it. Though I had no trouble dodging it, you'd have to put that down as a negative. Unless you're unusually fond of it. I'm not, particularly.
* It's not paved (would be awesome if it were), but the packed-gravel surface is fine. I don't mountain bike because I don't care for having my teeth rattled and my head moving around like a Bobby Bonilla bobblehead. I was fine on this trail. The roughest, rockiest surface is a brief stretch after you come out of the woods going on to the Northern trailhead. But it's short and you could actually ride that stretch on the road that runs parallel to the trail at that particular point.
* 20 miles of continuous riding on a paved trail would take, at least at my usual speed, a couple of hours. You'll probably naturally go slower on this trail, so it will take significantly longer. My trip actually lasted five hours, what with lots of stops for photo-taking and restroom breaks and such. So you can definitely make a whole afternoon of it, if you're of a mind to.
* At the northern trailhead right beside the little church which now serves as the terminus point (and has restrooms), there is a small house with a fenced-in (thankfully) yard in which there are about a dozen dogs. They will bark loudly and continuously at you as long as you're around. Unfortunately, the men's restroom is closest to the house (very close, in fact), and so I got a real earful. Though, thankfully, that's all. Maybe they should put one of those electronic gizmos near the church that supposedly stops dogs from barking. If those really work.
* Heavily shaded virtually the entire length, the major exception being the town of Elkmont. Even that isn't very long.
* Very decent restrooms at both trailheads, and also in the depot in Elkmont (though the restroom there has no sign on the door. It's on the south end of the depot.) More than ample parking at both trailheads, and Elkmont. They obviously made space for horse trailers. Convenience store in Elkmont. Stroll over to it, maybe buy yourself an RC Cola and a Moon Pie, and enjoy while sitting on a bench at the depot. There are two nice little rain shelters on the trail, one on the south leg, and one on the north leg.
* I've posted photos beginning at the southern trailhead, Coffman-Mitchell Park, which is mile marker 0. There are photos at every mile marker, as well as particular points of interest. The mile markers on the trail are quite nice and clear (I've been on other trails where they were confusing) and posted not just every mile, but every half-mile. Very nice.
* Though I see others have commented about an uphill grade at some point, I honestly never noticed anything like that. To me, the whole trail was nice and flat (brief, minor exception near the Northern trailhead.)
* Good for Richard Martin for pushing to get this trail built. Maybe one day it will be extended south to Athens, and north into Tennessee. Mr. Martin, I salute you!