Note that with any in-walls placement is probably the biggest performance factor (limiter) because the speakers are smashed against the wall (limiting performance), plus once they are placed you can't make adjustments. So in some ways it matters less what you choose (within boundaries of price/class reasonableness anyway) - the biggestest limiting factor in the whole system will be the fact that they are in-walls, and you can't get around that. That being said, the reasons I chimed in:
1) Note that the Magneplanar on-walls are a possible exception to the you-can't make adustments point.
2) You may want to consider the in/on-wall speakers from Axiom. They eliminate some of the smashed-to-the-wall point (while still providing decent internal volume - unlike most of the pure on-walls that are usually marketed to go with plasma TVs).
3) I'll put my second to Gonk's point that I would use an in-wall subwoofer as a last resort. Selection is limited, and the placement point may also be an issue. And because you usually have a lot of placement options with a subwoofer (and it sounds like you have some) the clean-look reasons for using in-walls is less in the case of a subwoofer.

One more point I guess - I did have have Polk in-walls and they did sound very good, in perfect circumstances (e.g. I had a Polk TC65i in-wall center channel blairing straight me through the audio-passing projector screen....that sounded great because placement was not much of an issue in that special case).