It is extremely unlikely to produce euphoria. Any euphoria you may feel if you abuse it is just chance I'd argue. It will produce sedation at lose doses but at higher doses it will actually make you more restless which will get even worse if you take more. From Wikipedia (
http://en.wikipedia.org/wiki/Diphenhydramine#Recreational): "Considerable overdosage can lead to cardiac arrest, serious ventricular dysrhythmias, coma, and death."
I disagree that diphenhydramine doses that led to hallucinations (I believe you can reach this point without uncontrolled delirium) cannot produce insight. If you like delirium of the type produced by altering your muscarinic receptor (for example like that produced by Amanita muscaria), i.e., an anticholinergic that is an antimuscarinic, then it's pretty much what it is.
I don't think this a pleasant delirum, but it's not that different IMO from many other deliriants. Delirium is what it is. There's nothing happy or sad about it and if you get to a stage of delirium, it implies total loss of control unlike with a psychedelic like LSD or psilocybin where you do have a lot of control (unless you really overdose on it and even then with a sitter it's able to control your states whereas with the anticholinergic delirium I don't think it s controllable).
You can hallucinate with diphenhydramine if you like by slowly increasing your dose (in the minimal unit increments available and you're comfortable with) and waiting to see what happens. Let's say you're at 100mg (which is a small dose unless you're sensitive to antihistamines so you should start at something like 50mg or 25mg if you want to be really careful), stare at a particular object intensely for a while. You should see patterns develop and movement of these patterns up and down. I believe this is a strong characteristic hallucinogenic state of antihistamines that you reach before you head off into delirum. You probably should stop at this dose. If you go slightly further up you'll see patterns take shapes that are beyond your control usually but some suggestion can help focus those patterns. You REALLY should stop at this dose. If you go further up I believe you'll start making your way to full blown delirium with uncontrolled hallucinations and likely your local ER (I say this not to scare you or anything but just based on my experiences and my knowledge about the neurochemistry of the muscariinic receptors).
I do see an analogy to DXM here. DXM highs aren't exactly high or pleasant but they have their value if done once or twice in a life time. Getting high off of diphenhydramine on any kind of a regular basis is associated with a huge number of health problems (google).