We have used it for years. All of us - husband, both sons (started at age 6) and me.
When you do it, just don't forget to keep your mouth open and breath thru your mouth - that was the best piece of advice I was given. After you do it the first time, you will understand why. It will feel weird, almost like going under water (thus the breathing). Also, have a kleenex next to the sink, so you after you blow your nose in the sink and get all of that snot out of your head (and you will get a lot of snot out of your head), you will want to wipe off your face. Alternate each nostril at least twice.
Yes, you need the saline packets that are for neti-pots - you can get them at Wal-Mart, Target, CVS, Amazon etc.
Once you do it, you will never want to take nasal sprays or decongestants again - we haven't!
When allergies are bad we use it 1-3 times a day. My kids ask for it when they are stuffy. My husband uses it in the shower so he doesn't have to be neat about it :)
It is totally worth it! You will wonder why you didn't start using it earlier than now :)