First off, what are you using it for. The sleep aid in those can, at high doses, be used as a deliriant/psychedelic drug. However, I'm pretty sure that to get a high enough dose, you'd have to take a VERY high dose of Ibuprofen (the general name for what's regular in advil.) Ibuphophen can cause liver damage at excessive doses over any prolonged time. I'd suggest reading up on Diphenhydramine and Ibuprofen on wikipedia. (It's a great source of information on how it will affect you, as well as information on the dangers of using too much.)
From what I can tell, Diphenhydramine has a lot of side effects that are not what you'd want. Overall, it's almost like a mildly hallucinogenic version of alcohol, description wise, but it also has some side effects that aren't very good.
From what I've read, you probably wouldn't want to do that. If you're serious about using that in a recreational way, I'd suggest locating another source of Diphenhydramine and possibly even separating it from whatever it's bonded to. However, that may require chemicals you can't get access to, and knowledge of chemistry you don't have.
All in all, I'd advise against it. You may become mentally addicted to that stuff. (sounds like you're right on the cusp of that right now). Being mentally addicted is not really a good thing (though it's easier to break than being physically addicted, it's still not fun to deal with, and in this case, could probably do some damage to your liver, and possibly other areas.)