My '01 4.4 (85k miles, purchased in Feb at 79k) seems to have been properly maintained by the previous owners, my intentions is to hold on to it for several years and get it up to 200k plus miles. I do all my own work, so its cheaper for me to put money into the parts than go buy a new or newer one and get hit with $600 month payments and 30-40 grand in depreciation.
I would expect to see prices for the early X5s to continue to drop down to the $10,000 -$14,000 level in the next few years then start to level off. I'm sure in five more years a beater X5 with a bazillion miles on it will be able to be had for a few thousand dollars (parts car!!), but I expect that on the whole they will hold value pretty good.

I'm basing my prediction on e39 values. The X5 is based on the e39 and shares many of the same systems. As the X5 is all-wheel drive and an SUV, I expect that values will remain a bit higher than, or the same as, comparable e39s.

Somebody can look up this thread in five years and see if any of us were correct!