Sure it takes up some of your review mirrors real estate with its LCD screen, but it offers up so much in return. Namely GPS, Bluetooth, and it can even work with your backup camera if you have one.
The screen is a touchscreen and has the GPS receiver built into the mirror right behind it. You can also use the Bluetooth for making and receiving hands free calls from your cell phone.
There is an SD card slot and a USB port for expansion, although no word on what exactly works within those two ports, but I presume the SD card slot is for upgrading the maps. No word on price or availability, but the GPS chipset is a SiRFStarIII, the OS is from MS, and the CPU is from Samsung.