I have had the opportunity to access the Steamboat Trace on multiple occasions. I believe it's approximately 26 miles from north to south, but starting at Peru is what I would recommend.
Peru, the home of Nebraska's first and oldest 4 year college (Peru State College) is a quaint community. If you ride on a fall Saturday, you may just find yourself a short distance from the Oak Bowl and a chance to watch some great college football.
Services are available in Peru at Cotty's Restaurant, grocery store for supplies, a bank, etc. . A parking area with a spot to camp, water, and restrooms are right next to the trailhead.
If you aren't an expert rider, start the day heading south to Brownville, about 7-8 miles from Peru. You'll hugh the soaring bluffs for a few miles, and will glimpse the Missouri River itself just a few feet from the trail. Before the river cut a new path away from Peru, it's said that Lewis & Clark noted these very cliffs in their journals because of all of the bird nesting holes that are evident still today. Nice to have some history in your mind as you pedal on!
There's a conservation trail (walking only) that's located off this section for a respite, but the trail then crosses the floodplain and meanders towards Brownville. At Brownville, a great place for lunch or supper is the Lyceum or stop at the winery and pick up a bottle for a picnic on the way back!
So when you make the loop, you've got 14+ miles under your belt by lunch or so. If you're feeling good, you can ride toward Nebraska City in the afternoon and put in another 38 miles roundtrip.
Many of the comments concerning the condition of the trail are correct, but wrong at the same time. Because the trail is located along the Missouri River, it is damaged or flooded when the river is out. This occurs on an infrequent basis, but it does occur. Once the water recedes, the NRD staff gets in there and does a yeoman's job of putting the trail back in shape. I've seen fliers organizing the local community to pitch-in on designated days for trail cleanup as well, so I think people do what they can to keep it up. But if there's been a big rain a few days before, expect some light damage or soft spots (especially near county road crossings). My daughter and I typically use our mountain bikes and never experience more than a bit of mud on the worst days. One has to be reasonable though and understand that this is a rural trail in a rural area, that runs through very soft Missouri River soil. Despite lots of work, it sometimes appears that it lacks maintenance, but of the ten times I've been on it, I've seen workers about five times. Frankly, I'm impressed that the small cities and NRD are doing what they are so I always try to encourage them and make them know they're appreciated.
If you're considering this as part of a larger biking trip, I take this on a day and then strike out on the Wabash in Iowa for a while. Great variety and I wouldn't give up the tremendous scenery and nice people around Peru and Brownville for a longer stretch. Cowboy and several other trails developed by the Nebraska Trails Foundation make Nebraska a mecca for biking anyway, and if you're near Peru you wouldn't have a complete experience without stopping in.
Use is kind of a fickle thing. Summer and late fall, we're scooting over all of the time for packs of bikers and nearer Peru and Brownville, it's full of walkers and runners. Of course, we actually like the trail because it's less crowded than others in-between urban areas, so I guess it's a matter of perspective concerning whether it's used much.
If you live in the Kansas City - Omaha - Des Moines - Topeka region, I can't think of a more fun trail for a day trip. Give it a whirl and see if you share my opinion. Recommend the onion rings at Cotty's!!
Only caveats are that the trail is closed during deer hunting season I'm told. I'm guessing the way to find out when that is may be by contacting the Nebraska Resource District for the area. Hasn't come up for me as my trips haven't included November and December.