I have a mixed feelings about my own characters being in IC relationships, but at the same time it does present some advantages with the disadvantages.
For instance, it gives you a ready made RP partner and set of goals for your character to pursue. On the other hand, it often leads to getting into an RP rut. You get so comfortable RPing with that character that it can get harder to RP with characters you can't read as well.
One irritating side issue that usually comes with it though is other RPers running off just as soon as your character's significant other comes along. Not every RP between couples is a private RP

, especially in a public venue. However, it is easy to slip into ignoring characters since you are already in the habit of your character responding to their S.O. first.
Other potential wrinkle is when the player behind the S.O. has to leave for RL reasons. That can put a strain on the story as you struggle to reconcile the loss. You are usually left with an off stage S.O. or a contrived break up. Nevertheless, it is sometimes unavoidable. It's one of the reasons I tended to avoid my characters being in relationships unless the connection between the characters was so compelling that it would be an obvious contrivance to avoid it.
As a side note, pre-arranging past relationships, or attempts at them, with other characters can also make for fun "in the present" RPs too.

Overall, I think it can be useful as long as it stays a facet of your character and not the
grand sum total of their being. Relationships can be used for highlighting each other's characters and can lead to character development, even for periphery characters... just as long as the characters stay mostly inclusive.

What do you all think about IC relationships?