I don't have in-walls, but I do have in-ceiling. They image great, and sound awesome. In-ceiling, in-walls are just like any other speaker. They all sound different, and some are more quality then others.
I would recommend "sound enclosures" for atleast your mains. Surrounds are up to you, I just used brackets.
Take a look at my Webshots pictures. You'll see the enclosures and speakers.