My X3 was perfect for all three years of the main warranty. I paid for the BMW extended warranty going into the fourth year and am VERY pleased I did.
The only fault so far was in the Satnav screen. A £2,000 plus labour item that only cost me the £250 XS (and the premium).
That said, I have paid around £1000/year for my Mercedes extended warranty for six years and never called it into action.
As "ronk" says, it's all a gamble. It's one I'm happy to buy my way out of though as fixed costs are better than a heavy hit (or two).