We have a 2002 3.0i and *knock on wood* haven't had issues. We take care of it pretty well doing things like changing the oil every 5K kms and stuff like that. We're planning on keeping it unless we fall into some money and can jump into a Porsche Cayanne S. I know of one 3.0 X5 that *popped* the motor but that's because it wasn't maintained at all. MEchanic said it was lucky if it ever had it's oil changed. Ours has been reliable and we have so far REALLY enjoyed it.
Current : 2013 Mini Countryman JCW
Past: '88 E30 M3, '69 BMW 2002 Ti, 2000 328i, Audi A6 TT, BMW X5 3.0i , Audi A4, Mini Cooper Clubman S 1.8T