I've ordered the PDair aluminum case as soon as it was available. I've got the black version - imho it looks way cooler than the "plain" aluminum one, but marks and scratches are much more evident.
I've used a PDair aluminum case for years with my HTC Wizard and with both smartphones I've never noticed any degradation of signal.
With the N900 I'm also using a Clear Protector Nokia N900 Extended Protector and it doesn't interfere with an external case.
The aluminum case adds a bit of bulk to the phone, but it gives a much better protection than everything but having it locked in a drawer My HTC fell from ~3 m (10 ft) and it got no damage at all. Even the case was still in good shape...