This product works great. The 35% solution is much stronger than the other versions, and hence, you don't need to leave it applied for so long. The unofficial directions I received with my order said you can leave on the 35% Opalescence for up to two hours. However, the PDF instruction sheet that you can download directly from the Opalescence website, says specifically to not leave the 35% applied for more than a half an hour.
I would recommend that new users keep the solution on for less time in the beginning, gradually building up tolerance. The PF version also contains fluoride to help control sensitivity.
When first using the 35%, I left it on for about two hours, and I did experience some sensitivity. The sensitivity went away in a day or two, no problem. After using it a few more times, I am now able to leave it on for about two hours with no sensitivity whatsoever. I cannot imagine trying to sleep with trays in, because you definitely don't want to swallow any of this stuff. That's one of the main benefits of using the stronger 35% solution: Reduced tray wear time.
Since you won't be licking your lips or swallowing for a while, I like to apply a bit of chapstick to my lips, and have a good drink of water before getting started.
The PDF instruction sheet from the Opalescence website also states that you should use approximately one third to one half (total) of a syringe worth of solution to fill both trays. Good to know.
To target specific stains, I add a little extra to that spot on the tray, and I apply a bit of solution directly to my teeth. The syringe makes it easy to put the product right where you want it. Don't bother putting solution on the back of the trays, because no one is going to look at the back of your teeth.
The date codes were explained to me this way: The numbers indicate a month and year of expiration. For instance, V023 indicates that the product expires in February of 2013. The seller I bought from was Jawtooth.
As far as trays go, a standard custom mouth guard worked well, though obviously custom fitted trays from your dentist will work better. The mouth guards can be had for about a buck a piece in retail stores, but if you buy them on Amazon, you get three for under two bucks. Nice to have an extra in case of a misfitting.
The results this product produces are fantastic. I had a dentist ask me if my teeth were caps. (they're not) No kidding. I will use this product for as long as Amazon sells it. There is no reason whatsoever to waste your time and money on store bought white strips, solutions, toothpaste, or mouthwash. This product really does whiten your teeth much better.
Hope my experience makes using the product a bit easier and less intimidating for you. I'll update if I think of anything else helpful or relevant.