I think the main danger of NRE is to existing relationship. Really, due to the mono-normality we're raised with, even poly people might leave an existing relationship due to the NRE with a new one, rather than try to make both work.
There is also a clear danger of neglecting the existing partners, even when you are poly and aware of the NRE's risks. I think having supporting partners really help, so that you can share that NRE with them. In my case, it helps that Rag has an interest as well, so we can relate to each other more easily in that way.
I would say I'm reaching a part where the NRE is still strong but I'm starting to see more flaws in Sean. Which is very good. See, before that, because I seemed to only see his good traits, it could make Rag less "interesting" in comparison. While I still have a strong ORE with Rag, the thrill of NRE is different.
Now I feel like I can be a bit more impartial, mostly about their differences. There are ways I am more "compatible" with Sean and, sure enough, ways I am more "compatible" with Rag. And lately, I have noticed the importance of the ORE: how we know each other, understand each other, have similar values or know how our values differ and respect that...
I'm still in a phase of exploration with Sean, and I'm realising that we have many more cultural differences. While Canadian, Rag is very "European" in many ways. Sean is much more American.
Difference can be good and interesting, but they can also come as a shock. I'm enjoying being at a place where I can enjoy the differences between Sean and Rag, and appreciate both of them more for that.
Really, it first happened when I met Sean. We clicked very well, thanks to the NRE, and being apart again was very hard, as happy as I was to be with Rag again. The next few days were very harsh, because my interest (a coworker of Sean's) changed jobs and states before I could make a move, and I was worried of having missed my chance. Add to that missing Sean and I was in a very weird, conflicted place.
I was very worried about neglecting Rag, but later we spent very good quality time together and I could appreciate how we don't have to do anything "special", how just being together is soothing, how we understand each other so well due to years of sharing each other's lives.
Then I reached a new point in my relationship with Sean. While we used to talk every day in some form, we started talking less, partly because he was busy, partly because I was spending time with Rag when Sean wasn't busy, and partly because even when Sean wasn't busy and I wasn't spending time with Rag, I wanted some time for myself.
I feel that helped me get over the addictiveness of it, and enjoy the good parts of NRE without stressing too much about the bad ones.
Wow, I guess I made a longer post than I first intended. Either way, I think NRE can cause a balancing act, but I'm enjoying it very much.