I very nearly rolled a DP/EM when I first got a hold of the set. At that time though I hadn't played Mental either, so I rolled the Dp/MM when GR first came out.
I have a lvl 50 Ar/em though so I can comment a little.
Energy is a great secondary for any blaster, however those with cone attacks I think will get a little more out of it.
On mine, I took Boost range by accident, but it turned out to be amazing when paired with any cone attack, dual pistols has one, plus you can fire from longer distances.
Another possible reason it meshes well with pistols is Power Build up. This will boost the secondary effects of your cyro rounds, indeed it does make some mobs crawl. I am unsure if it effects toxic rounds the same way, but I believe it effects your melee attacks for extra mez on those disorents.
Also, as you know, Energy has some great Melee attacks, a lot of damage to be had.
I also have a lvl 50 fire/em, and I agree, those are fun! dp/em should be just as fun, especially with that speedy tier 9 
Combat Kangaroos, Justice Server. First 50's
Jirra Roo Plant/Storm/Stone/Musculature Controller
Combat Kangaroo Rifle/Energy/Mace/Spiritual Blaster
Kung Fu Kangaroo Martial Arts/Reflexes/Body/Spiritual Scrapper
Tribal Arc Shield/Elec/Mu/Spiritual Tanker
I have one, but I only went DP/En to ensure I could avoid redraw and for set bonuses. Between Conserve Power and Stamina, I feel safe running leadership toggles on the guy, and when we get inherent stamina I'm actually going to be lost and confused as to what to do with those extra power choices.
I have one at 30 and it's a fun combo. I did skip the melée attacks to this point (blasphemy on an EM blaster, but I'll be taking TF though). I mainly wanted to try out the DP attacks and have that the central focus of the character, so have taken all of those plus build up and power boost. I don't find endurance usage to be unmanageable, but conserve power would be nice to have.
I'm just not sure what you would drop in DP to pick up some of the melée attacks (taking into account travel, hasten, and aid self).
If you like DP/EM, you may also like DP/ELEC. I have one at 50 and I find it is one of my favorites. It plays similar to EM, you have a few melee heavy hitters but it also has the added bonus of a damage aura and power sink. Usually when I run in, pop hail of bullets with lightning field running, there isn't must left standing. I pick off the one or two remaining with executioner's shot or a melee tap.
I have shocking grasp and char from the epic pool, I spam them as available and it helps me control any bosses I need to beat down in close range. Power choices can get kind of tight but I19 should help a bit. I may get assault since it requires minimal slotting. Power sink takes care of any end issues.
So far this is a blast to play (no pun intended) but the character is not really high level, anyone playing this higher level that can give advice? I'm thinking its along the lines (power wise) like the Fire/em I played? some range, some blap?
Edit should say Dp/Energy