The design is pretty poor.

In the E39 & E38 the regulators arn't all that bad, maybe slow or they just stop one day.

But the X5 is the only one that will actually DROP the glass when the regulator gives, doesn't slow down or give a warning, it drops it like it's hot. (Click . drop . -speak eloquently-)

End rage.

I bought on ebay, bavauto.com, dealer and none seem to be a durable as the ones at 1aauto.com, I am not affiliated with them, just seems to be warrantied better and so far so good. Stock regulators typically last 5 years. You are exactly right, they drop without notice and I get enraged too, i was at a hotel and I told the valet NOT TO USE THE Windows, what did they do when they brought it back, the drivers window was down and they said I never told them. They bought me a dinner as it was on camera and I had a witness but still. What a pain, I live in AZ and I think it is worse here with the drying out of anything rubber or plastic.
Now I just replace the window regulators every 4 years. 1aauto seems to have pretty good prices and warrenty. I found them through ebay actually now that I think of it lol.

my drivers side is going too on a 2004/11 e53 3.0d and yup not just an x5 issue, bmw e46 my wife has ALLLL the regulator clips broke, replaced with plastic moulded ones recently but they dont seem to be a good a fit as the originals and the little red clip that holds the glass the guides cannot be tightened too good ......oh man am dreading ripping the x5 door apart.....jeez