OK, most, well really all External Bottom Bracket systems are all the same, basically you take a aluminum cup, press a bearing into it, thread it into a frame, slide a spindle attached to one cranks arm through the bearings and bolt the other arm on the other side trapping the whole thing together. But there is one exception. The SRAM GXP Bottom Bracket. The difference is in how the cranks are held in place.
Really it's just a simple ridge in the spindle that with the use a seal that has basically a washer built in the Non-Drive Side Bearing's inner race is trapped or pinched holding the whole system in place.
WHY? you might ask? What are the advantages or disadvantages? Well the biggest advantage is the lack of un even inward pressure on the inner bearing races while the outer ones are basically pushed outward twisting the contact points of the actual ball inside the bearing. By trapping just the inner race of one bearing the races stay "Square" to each other allowing for a more even wear. Also it balances the job of the two bearings. under normal use the Drive Side bearing take on the extra job of not just keeping it all going round and round, but also fights the forces up and down (you pedaling) which both sides deal with but also the forward back forces (chain pulling it back while you are pedaling it forward) that only the Drive side has to handle, and it's a biggy. By having the side to side motion strictly on the non-drive bearing it's one less job the drive side has to deal with, and also the bearing basically floating on the spindle the balls can stay aligned better with both races making it wear less, and in effect last longer.
Another advantage is the ease of installing chain guide and other Bottom Bracket Held Accessories. The Drive Side Bearing is actually just "floating" on the spindle, so you can add something behind the bearing cup which would normally change the chain line and other annoying affects. But as long as you don't add sooo much that the bearing actually comes in contact with the drive crank arm, you're good to go.
And talking of Chain line, the other advantage is you can easily adjust your chain line by adding (or subtracting) spacers to the Non Drive Side Bearing Cup. .. Add one spacer shifts your chain line in 2.5mm. You just need to make sure you don't bring it in TOO far and have the drive bearing seal bind on the crank arm.
Now the only real disadvantage is the install, and honestly it's no more difficult that Shimano or any other, But you do need to make sure the Non Drive Crank Arm is installed properly and is pinching the bearing race tightly. Where the interface of the crankarm is so tightly fitted the littlest dirt on the spindle's spline or in the splines on the crank arm you could end up with the whole system not getting tight enough and end up with play/slop in which case you need to start from scratch, pull it all apart, clean it well and grease then re-assemble again.
One other Disadvantage is actually nothing wrong with the system, but us, the people who install and or use the cranks. Many people see the space of the spindle showing between the bearing seal and the Drive crankarm and or Spider. The space is normal and is actually a good thing as it gives you room to add those things like Bottom Bracket Held Chain Guides and such. But many people think it's wrong and try to fill it or just plan screw with it. Just so you know, If you ever install the SRAM GXP Bottom Bracket, the Drive Side seal SHOULD NOT!! come in contact with the crank arm or spider, a small gap there is a good thing... just look at the seals, the drive side seal isn't designed to have any area of contact on it's face.