It depends on the individuals, their maturity, and their attitudes towards sex. I've been in an open relationship for two years now (and it was closed for 5 years before that), and have no complaints whatsoever. In actual fact, I would say our ability to communicate has improved, and we spend maybe a bit MORE time together than we did before - it's especially fun comparing notes, and it's given us plenty of ideas to bring into our own sex, as well.
Open relationships do also need to be balanced, in that both people are actively into the idea, and make sure to get out there and look for partners, otherwise major imbalances and issues can result. It can also be difficult at first, because you have to work through a lot of deeply engrained issues in regards to jealousy and possessiveness, but if you are a reflective sort who analyzes your own reactions, then this is totally something that can be worked on - and soon enough you recognize that the bond you share with that person is not the same as you share with others, and that (at least in a good relationship) it is only tangentially related to sex. Then it becomes easy... and, IMHO, emotionally more healthy.
Can it go wrong? Totally... but 50% of marriages end in divorce, and others end up being unhappy marriages that should have ended - and that's not even counting all the relationships that don't make it that far at all... so it's not like closed relationships have a great track record, and I know plenty of open couples who have been together for years/decades. Does it work for everyone? Of course not, but then again there is no one-size-fits-all for anything in life (and anyone who tells you there is is probably too deeply buried in their own worldview).
Any questions, feel free to ask.