Twice now, actually. Both long-term relationships.
Stuff that went well
As /u/PlayerofSax41 mentions, ENFPs are great for helping ENFJs cool down. ENFJs overload themselves frequently (guilty), and there are few people better than an ENFP to tell you, "Who cares, let's get drunk." I really appreciated that about both of the ENFPs I dated.
ENFPs can help ENFJs be more spontaneous. I'm great at long-term stuff, but living in the moment and deviating from plans can be harder for me. Both ENFPs helped me with that, and it was really healthy.
I think that ENFJs are maybe the best type for ENFPs during a crisis. Not to toot our own horns, but ENFJs can actually solve the problem without sacrificing emotional understanding or the need to socialize. I think the ENFPs really appreciated that.
They get us. At least, they get us in certain ways that aren't common. It was great to feel understood; ENFJs and ENFPs, in my experience, are moved by a lot of the same feelings and events, and that made for very passionate and intense relationships.
Stuff that went less well
Planning. In both relationships, the ENFPs absolutely hated planning, including planning for mutually-agreed goals, like moving, traveling, jobs, etc. Despite trying to share the planning/make it fun, I ended up picking up most/all of the slack. The real problem, though, was that the ENFPs would then feel like they didn't have agency in the relationship, since I ended up doing the planning. Vicious cycle.
Ambition/work. The ENFPs were pretty objectively bad at self-motivating to accomplish even their own tasks, whereas most ENFJs I know (including myself) are fairly ambitious and motivated. Two problems here: first, I ended up (in true ENFJ fashion) taking on a lot of their work to "help them" get through things (read: enabling laziness). Second, the 'accomplishment gap' began to stress out the ENFPs, even though it was a total non-issue for me. I think the ENFPs associated "success" with "structure" (not entirely inaccurate) and freaked out.
Reliability. Try to hold an ENFP to their word. I dare you. It isn't pretty. I never considered myself a person who cared about punctuality until I was shocked into caring about it by dating ENFPs. Flaking on plans was also a problem - ENFPs will regularly "not feel like it" and drop a social commitment on a whim (dinners with friends, events, etc.). ENFJs care about social harmony and really value loyalty and trust, so this was big for me. (FWIW, I've also noticed this with ENFP friends.)
Ethics. Both ENFPs had a really, really hard time writing people off for doing shitty things. Abusive family members? "They're family!" Abusive exes? "They're in a bad place!" Friends bullying people? "Well, they're nice to me." The less-constructive parts of my ENFJ brain started asking, "How much is their affection worth if they still have affection for people like that?" The ENFPs, meanwhile, thought I was too judgmental.
In general, I'd say my experiences dating ENFPs weren't bad, just... unsustainable. I love excitement, and I love adventure, and I love not knowing what's around the corner sometimes, but at the end of the day, as an ENFJ, I want to wake up and know that I can count on my partner. That just wasn't something I could say in either relationship.
Sorry for the rambling; feel free to PM me if you want to chat more.