I use in-ceilings for my surrounds. Both the Vife and I enjoy the sound. However, I have 12" floor standers for my main L&R, and a dual 6.5" Sony center, so I cannot speak for the value of in-wall/in-ceiling for LCR.
I also throw parties several times a year, so I use my mains at loud volumes quite frequently. Even though they are large, I can move them, angle them and remove(as needed). Whereas with in-wall, they are fixed.
The same goes for my center channel. I had it above the TV, but I finally re-arranged my set up and now have it below the TV. Honestly, I think I preferred it above the TV, but since it is an actual unit, I have the ability to move it, whereas an in-wall CC is permanently fixed.
I have not seen any schematix on placing in-wall/in-ceiling speakers. I have seen the Dolby and THX layouts here and on the interwebs, they are readily available, but they do not take into account for in-walls etc...