In wall vs regular speakers?

I agree with some of the other posters but allow me to share my experience:  
I have a tower 2.1/5.1system consisting of B&W 802's, HTM-1 center channel and CWM-DS8 in-wall surrounds, and ASW-800 subwoofer.  Another 5.1 system consisting of Klipsch R-5800-W (L,C,R), R-5650-S in-walls, and RT-12D subwoofer.  Also B&W CWM-8180's in-walls in a second zone for two channel only.

With the two systems for 5.1, I can honestly say that I could easily be satisfied with either (don't tell my wife) on 5.1 material.  Of course with two-channel the 802's are in another stratosphere.  But in both systems the quality of amp/pre-amp, or receiver approach plays a huge role in bringing out solid 2-channel performance.  I'd say in both cases separates is the way to go, but in the 5.1 Klipsch system using a quality receiver is quite adequate.

With the B&W 8180 two-channel system using a Vincent SV-236-MK2 intergrated I would say aspects of depth and presence are there... Just not at the level of the 802's, but very satisfying.  Coincidently before the 8180's were installed I used Klipsch R-5800-W's with very close results.

In my case the in-walls that I’m using fire in both directions in dipolar configuration, and have a very wide dispersion of sound... Even in the case where one room has an open side to an ajacent room. For the enclosed room side placement is just as effective as rear only install. Stereo center imaging is a touch more pronounced with the rear install, but both placements acheive outstanding full/ambient surround.

I initially had on-walls in both setups but for space, or a more stealth-WAF decided to try in-wall in the open room. Then once I heard the performance and felt I was missing absolutely nothing, decided to switch in my main listening area. Honestly can’t comment on monopole designs as I’ve never used them.