Generally I believe they are reliable as any later generation e46 since they are based on that chassis (by 2008, most of the bugs have definitely worked themselves out already).
I think all e83 X3s are made in Austria at the Magna Steyr plant which has won multiple awards for their quality control. At least get them before the ones that are made in the US ;-)
I bought my wife a CPO 2007 X3si and we love it. Her initial reaction was that it rode too rough for her liking (she came from a Honda!). However, she loves it after an initial week or two of getting used to. We drove a 2006 and it was so much worst, after the 2007 LCI (mid-lifecycle-facelift), the 2007 and onward X3s where much smoother riding than the 2004-06 MY.
If you're getting an automatic transmission, then this is the only thing I would be careful looking out for. I've read a lot of problems with the AT on the X3s and sometimes related to its "auto learning" abilities. Do a search online and you see that the general problem is that it hesitates when shifting up at times. Compared to my e46 coupe, its definitely not as silky smooth, compounded with a higher vehicle and rougher ride, it makes the problem more noticeable. If you're a heavy footed driver, you're likely to not notice the hesitation as it upshifts when you stomp on the gas. It's the lightfooted drivers (some women) that will notice when they lightly apply gas, the AT tends to upshift later than expected. It doesn't seem to affect all vehicles. But I have noticed that ours does "hesitate" or is slower to respond than expected - but nowhere near some of the horror stories I've read on the forums. There has been BMW service bulletins that address this issue, but it only seems to solve the problem in some cases - overall they don't think the problem exists. Sometimes resetting the AT learning resolves the problem for a short while. Anyways, I wouldn't let it deter you, I was really worried about it when I bought mine a few years ago, but its not really a big deal now (at least the AT we got).
Good luck. I love it because its the closest family vehicle I can get to my daily drive! It's also the only SUV that drives like a car so there really wasn't a choice for me. My wife grew to love it and now can't drive anything else.