This episode wasn't complete filler, since it did result in the oft lauded reunification of the Special Most-Awesomest Final Five. But Ellen's return was more than just a return to the ship. It was also a return to Ellen, which sank this very much into filler territory for me.
The true definition of filler, IMO, is an episode that essentially sets up no really new tangents, nor answers any existing ones. This is quintessentially that.
I'll give a more in depth review later, but suffice it to say, this was quite average for nuBSG.
That was one of my big issues with the episode that dragged it down, for me. Last week they seemed to be setting Ellen up for some real character development and moving beyond what she was before. And tonight, in very short order, she was right back to her old as if nothing had changed at all. It really was as if there was no real point in making her the fifth-- other than adding some cheap drama to the Tigh/Caprica-6 plotline.