I had a 2004 X5 4.4, sold it last year after the CPO ended. I loved that truck, but I am pretty sure they changed the model in 2006. Drives great, has the feel of a truck, not a wheenie car-based SUV. Gas mileage was rough, 14-15, but I drove probably 65% city / 35% highway.
The only issue I had was that withing 2 months of my CPO ending, I had $1200 in things I had to fix. Only big issue I had in 3 years (and they may have fixed them by now though) is the rear stabilizer - it has some bug in it where it will claim to be going bad, but doesn't. I took it in twice for that, was told it was a common problem. With most luxury cars, you only have to take it in once per year, and until the regular warranty runs out, it is free, then the annual CPO checks are $50. But, I had the car for almost 4 years and I had it in the shop at least 2-3 times per year - they are very quick and easily give out loaners or give you rides, and it can be an inconvenience, but was worth it.