I have had it numerous times over the past year. It was different at the endodontist than at the oral surgeon. The endodontist cranked it up pretty high for the root canal I had done. It was my first nitrous experience and I didn't know what to expect. I guess it was a little too high, because I was almost completely out of it. I could hear the endodontist and her assistant talking but it was like little blips and unless I tuned in, I really couldn't follow along. I didn't feel a thing, but didn't like not being as aware. I mentioned it and for the following appointment (she couldn't get all the canals at the first one), she turned it down a bit. That was much better. I felt drowsy. I felt the heavy legs like you said, but kind of felt inbetween wakefulness and sleep, like when you are dozing. I was relaxed.
At the oral surgeon's office, for the first extraction, he had it pretty low, and I kept waiting for it to take effect. I just felt a little sleepy and the heavy legs, but not relaxed at all. He came in and asked how the gas was working and I said I couldn't really feel it. He turned it up a bit and then I was able to relax.
He also told me to give the thumbs up or down to turn it up or down during surgery, which helped me feel better about it.