I have a somewhat lengthy list of experiences w/ combining the two substances. First and foremost, I'd advise against taking doses above maybe 75-100mg of Diphenhydramine altogether, whether in combination w/ something else or on its own. A common side-effect of Diphenhydramine is restless leg syndrome.. And when taken in excessive doses, this effect hits you hard, and I find it to be extremely uncomfortable and it tends to kick in just when you're attempting to give into slumber..
But anyways, back on topic. In my experience, when you take maybe 25-50mg with a few beers, it definitely increases the effect of the booze. But also the sedative qualities. So your chances of passing out against your will are greatly increased. So I typically only do it when I'm short on beer, take 25mg w/ my first brew, drink a couple more, N if that doesn't do the trick take down another 25mg w/ another brew. Drunk and tranquil feeling.
So! I'd say it's worth a shot, but you should really pace yourself and avoid taking huge amounts of Benadryl.. it could cause oversedation and/or shitty drunken RLS. Cheers! :D