I was in an 'open relationship' for about half a year. Being based in London and her in Leeds, it kind of made sense at the time, considering we liked each other, enough to want to care for each other and talk a lot, but not enough to completely remove other possibilities that may occur and such.
One thing that was really important during it was that when I was up there, or she was down in London with me, was that it effectively was like a normal relationship. Everything we did those times was like a relationship, but when either one us went home or so, we still kept in contact by calling, texting, a few things on facebook and whatnot but we agreed that it was okay, if there was a possibility or temptation with another person then we allowed each other to pursue it. I know she slept with about 3 other guys in the course of that time, and I can't say I slept with as many girls at all, because I usually hate approaching girls anyway, but hate proper relationships. So that's why it worked.
Anyway, it ended because I think we personally got a bit tired of it, and moreso, I think, one of the guys she was with one time wanted her to get more serious with him and cut the open relationship we had. And I didn't miss it.
It worked out great, because we never had a single argument, we incredibly close at times and enjoyed many great experiences together, felt more than just '**** buddies' and was actually quite romantic at times. But it avoided all the stuff I hate with relationships; the insecurities, the arguments, all of it.
I can't say I'd have another one because even though it wasn't much effort, it was still some effort at times. Which is less desirable than being single.
Hope that makes some sense!