The current generation X5 has proven itself to be a decent off-roader. While not nearly as capable as, say, a Land Rover, the X5 has made considerable progress since its introduction back in 1999. Offered with optional AWD, the X5 can handle rough weather conditions. Called the "xDrive", the AWD system is something worth considering if you plan to take the X5 off the highway. While it's not the best choice for off road, it can still pack quite a punch in terms of performance and style.
Unfortunately, all of this comes at a high cost that should be taken into consideration.