Just want to share my 2 cents, as i have just installed two flush mounting in-wall speakers after searching various forums for help the past weeks.
Bottom line is, to my ears they sound phenomenal. They are a couple of Sonance Visual Performance 65. They are matched with a solid subwoofer and two surround speakers from SVS, but the sound (and look) is phenomenal when i listen to music alone using only L-R channel (+ sub). I have a rather large living room (like 60 square meters), and can literally rock it down without no problems or distortions.
Now, am i an audiophile or mega-expert? Absolutely not, and i am sure these rather "cheap" shallow speakers can't match the figures of mega floorstanding speakers. But i am certainly more than the average Joe: i have solid music background and consequent ear, and i do listen to music a lot, and i can tell you i am totally happy. For perspective, my mother has in her house two mega meridian speakers (i think they cost like 10x mines), is a classic music maniac, and can't tell any obvious difference.
Don't worry too much for audio you in theory cant get from in wall speakers, unless you are an audiophile you'll be absolutely happy.
Hope my experience helps, as i've gone through similar doubts for months