I know that Valentine One and Escort 9500ix seem to be the detectors of choice by most. I however, opted to go a different route. I also did not want a windshield mounted detector and wanted a more stealth approach. I decided to go with the K40 RL360 remote detector in my M3.
I mounted the front detector antenna behind the front bumper skin, just below the driver's side headlight. I mounted the rear antenna behind the rear bumper skin, just below the passenger's side taillight. Both cannot be seen or removed without removing the bumper skins. The speaker is attached to the passenger's side insulator panel below the dash, facing down to the floor. And the remote control has a visor clip that I attach to the map pocket on the driver's door.
The LEDs that K40 supplies with the unit are blue and very bright. Mounting is critical, because they can be hard to see in the day or blinding at night. I originally wanted to put them behind the left and right turn signal indicators in the dash cluster. This would be excellent during the day and night, but I did not want to void my warranty. I decided to install them in the ashtray pointing straight up. I can see them in the day and at night, they are not aimed at my eyes. They do tend to light-up the interior at night, but if it gets too annoying, I simply close the ashtray.
K40 requires this unit to be installed by authorized dealers in order to maintain the detector's factory warranty. I have a lot of experience over the years installing automotive electronics and after talking with the dealer, they sold me the unit for $950 carry-out, instead of the installed price of $1500. I did not opt for the laser detector/shifter package as those are illegal in OK and I haven't had much success with laser detectors in general.