Just my experience:
At a party, this college wrestler found out I was an Aikido instructor. He kept bugging me, saying he wanted to wrestle. I told him several times I wasn't interested, that Aikido isn't a game to me, and that how well it did or didn't work in a sport environement was irrelevant to me. So I let my conscious mind ignore him and started talking to someone else. Suddenly he shoots for my front leg. Dumb ass. Any of you folks know what men-nage is? It's practiced more in Iwama period dojo, but videotape has a wonderful way of blurring lines of style. Before I go on, I have to say in this situation that I think a knee in his face would have landed me on my butt. To much force against force by the time I got it up there; and indeed, his head was turned to one side, so it wouldn't have been very effective anyway. Anyhoo, I stepped the leg he was grabbing (my right leg) back, put my right hand on the back of his head, left hand under his chin, (palm facing my right, fingers up), and turned my hips 180 degrees to my right while rolling my hands over, taking his head with them. Looks really nasty, like I'm about to break his neck, but is actually pretty safe. His girlfriend screamed, he was stopped mid flip by his back hitting a furnace. Not hurt at all, but definitely aware of what was what as I kneeled on his chest with a hand on his throat, explaining to him that I really meant what I said about not wanting to play games.
Some caveats: 1) He wasn't a shootfighter, he was a wrestler. 2) He was probably a little tipsy. I on the other hand was hammered.
Another thing. An earlier post mentioned that Aikido only works when it's done really well. My experience doesn't bear this out. I was using sloppy Aikido succesfully after about a year of training. I think sloppy Aikido works fine against the AVERAGE attacker. In the dojo, we work against the most dangerous possible attacker: someone with no thought but trying to kill you, who keeps coming even when things don't go his way right off the bat. Most people you'll have to defend yourself against outside the dojo (on the rare occasions you'll have to do it) aren't well trained, and usually have are simply looking for easy prey.
Ne-waza: never had formal training with it, but I used to practice a little with a judo club that met before my Intro to Martial Arts class at the college here. Never had a problem applying Aiki principles on the ground. I don't have any desire to become too comfortable being on the ground, because it's a lousy place to be when the bad guys friends show up. I prefer to stick a finger in his eye and get up as soon as possible.