The systems that mount to factory rails are pretty straight forward in terms of how they are attached. Even so, for a simple two bar / four base system you're looking at about 10 - 15 minutes for installation and removal, a bit less if you have a helper who knows what they're doing.

You'll find a tool at this web site that walks you through the hardware selection process for either a Yakima or Thule system for your X-5: http://www.orsracksdirect.com/bmw-x5-roof-rack.html

Links to a variety of other Rack systems and sources are here: http://www.TheTandemLink.com/TLLinks.html#anchor810905

I pretty much left the roof racks on my '97 Suburban all the time (Yakima) as it was pretty much the tandem hauler. Often times it was preferrable to put the mud-encased off-road tandem up top instead of inside after a day on damp trails. However, given how long the roof was, I was able to use a three bar / six base system with single bike mounts for hauling tandems that gave it a low enough profile to sneak under the middle garage bay's 7' door. No way that would have worked with even an ATOC mount. Moreover, the newer Z71 Suburban's ride much higher and have a more robust type of rack system, both of which preclude the installation of a stock roof rack system. So, not having had enough time to machine any fittings for an alternative crossbar mount, we've just gotten by hauling the bikes in the truck and clean the removable carpet after the muddy bike is taken out of the truck.

This is a link to some photos and descriptions of our roof rack systems:
http://www.thetandemlink.com/articles/cartopping.html

I used a step stool and the rear seat of our trucks as my platfork when hefting the tandems up onto the roof of these vehicles. It was a bit of a balancing act in that being only about 5'8", I'd remove the front wheel of the tandem, then lift the tandem by the seat tubes up over my head (noting that they all weigh less than 50lbs), then would step up on my step-stool with a passenger door open and end up with one foot on the stool and the other on the back seat of the truck. Once I had my footing, it was just a matter of sitting the rear wheel in the wheel tray and then navigating the fork into the fork mount. A quick flip of the rack's fork skewer and the tandem was safe from falling over and then apply the rear wheel strap. Now, I'm not sure I'd recommend this acrobatic stunt unless you're really comfortable dancing around on ladders, surfboards, or skateboards.... a helper would eliminate most of the drama.

Of course, if you want to eliminate all the drama there are a couple receiver mounts that are worth looking into. While the Draftmaster (now owned by ATOC) is often times mentioned, the one system that I would probably look at if I was considering a bumper-guard bike rack would be Badger's horizontal mount.

http://www.badgerrack.com/tandem.htm

Badge also makes a vertical mount along the same lines as the Draftmaster; however, If you only have one or two tandems to carry their horizontal system is pretty slick. However, I'd most likely remove the rear wheel to keep the tandem in the slipstream of the vehicle as well as keeping it out of the elements. It also eliminates concerns regarding low-overhangs, the potential for your or a helper scratching or denting the roof or side of your high profile vehicle whilst loading or unloading bikes.

Lots of options out there; just take your time and do your homework as some of these systems can get really expensive. At last count I think I probably have four complete Yakima tower/cross bar systems and 8 bike mounts that have been accumulated over the past 20 years along with various kayak, ski, gear basket, and wind deflector attachments.