I can't say this is anything stellar, but it's exactly what you'd expect - boy gets harem, fanservice ensues, there's some fight-of-the-week as mystery bishoujo defends boy from monsters. Some will find this as more proof that anime is doomed because If It Has Panties It Must Suck, and some will be moderately entertained. I'll fall in the entertained category, if only because Himari says "nushi" and speaks in archaic Japanese.
Maybe it's just Spice and Wolf's influence (and Himari has the same voice actress as Horo the Wise), but when girls use "nushi" for "you" it gives them Skull of Mondain-level powers somehow (there's an obscure 80s gaming reference for you, although Google knows all as usual). Not sure why I think this is cool because I can't speak Japanese worth a damn, much less the Japanese of the Heian period, but it works. The male lead somehow forgot most of his childhood (another tragic case of Aizawa Syndrome) so I expect some drama towards the end as he remembers his past with Himari, and I'm sure we'll see the new enemy at the end of the episode joining his harem in a week or two tops.
Not expecting much from this, but I'll still give it a couple episodes - the plot seems monumentally generic but I'm easily entertained, so I'll keep watching for the nushis and Himari-Rinko cat-fighting if nothing else.