The bad thing about the number of people is that over 1/2 of them are drunk. We canoed it 4 years ago. I recommend starting early to avoid the crowds. It's a beautiful float.
I use to live in North Platte which is incredibly embarrassing to admit, and I've paddled the Niobrara more times than I can count and alcohol was a pretty bad problem for a while. One weekend we went some drunks at a campground were using a water balloon sling shot to shoot full beer cans at people floating by which is really dangerous and really stupid. But in the last few years they have really cracked down on alcohol, most of the camp grounds and outfitters have a zero alcohol policy and law enforcement to hang out and bust people for public intoxication. So it isn't as bad as it use to be. The biggest problem is with the huge groups of people that lash up a dozen or more intertubes and hog the whole river.
If you are up to it, I'd also recommend the Middle Loup River. Its nice like the Niobrara but a bit more challenging. Once you are comfortable with that, give the Dismal River a shot. Its got a lot of blind turns and strainers so it will keep you on your toes the whole time.
Here is a picture of a trip I did on the Dismal once. The paddle in the foreground was my dads, he dumped it in a strainer and his paddle got wedged in the wood and with the force of the water it bent the paddle. When we finally got it out we tried bending back straight but it broke, so we had to do a field expedient repair and it held up well. Now that paddle adorns the wall in my dads office.
