I tried it for the first time with DS last year when he was 4 1/2. I had purchased a sinus rinse kit for myself because of sinus headaches and noticed that DS had a really stuffy nose that no amount of saline spray and suction with the bulb syringe would get rid of. So I used it on him instead.
It worked well for him -- I had him lean over the sink and I gently squeezed the bottle, telling him to keep his mouth open and to breathe thru his mouth. Worked fine and I started gagging, almost vomited because there was sooo much nastiness that came out. He thought it was cool.
Since then we've had a few struggle convincing him to do it. We don't do it all the time, just if he's coming down with a cold or is really stuffy. He does well when he does it, but there are times when he throws a fit because he doesn't WANT to do it.