Lack of built-in flash on OM-D annoying or frustrating?

Yes, I find its omission really annoying.

Just for those occasions when a little fill-in flash is needed to give some facial illumination to a back-lit subject or some sparkle in the eyes, a pop-up is fantastic. These are almost always unplanned, spontaneous shots so the clip-on flash is not with me.

I use it only as a trigger for my Metz, otherwise it stays in the bag.