I've done the gel. The product I use is called NiteWhite. It's not one of the kits you can buy at the grocery store, I had to go to my dentist to get it. It cost me ~ $300 to get the molds made, plus enough gel for 3 rounds of whitening. I've done it about every 6 months since I got the molds. Once you run out of the gel, its super cheap. I don't remember the exact cost but its something like $35 for 3 more rounds worth. Very noticeable difference each time.
Edit: I just looked out of curiosity and the refills are much cheaper online.
[ame="http://www.amazon.com/Nite-White-Whitening-Latest-Product/dp/B000K5YCBI"]Amazon.com: Nite White Excel 3 ACP Z 22% Teeth Whitening 6pk Kit (Latest Product): Health & Personal Care[/ame] is 6 sessions worth for $19.99. I also saw some kits of the same brand that were 'make your own mold', but when I did it, the dentist actually fitted me with the high quality stuff they use, I have a mold similar to the things you see on the Invisalign commercials.
Edit: I just looked out of curiosity and the refills are much cheaper online.
[ame="http://www.amazon.com/Nite-White-Whitening-Latest-Product/dp/B000K5YCBI"]Amazon.com: Nite White Excel 3 ACP Z 22% Teeth Whitening 6pk Kit (Latest Product): Health & Personal Care[/ame] is 6 sessions worth for $19.99. I also saw some kits of the same brand that were 'make your own mold', but when I did it, the dentist actually fitted me with the high quality stuff they use, I have a mold similar to the things you see on the Invisalign commercials.