I bought mine CPO so it will be covered till 07/15 or 100K miles. I don't plan on keeping the car that long nor drive many miles, so it's good for me.
One thing to notice is that CPO is not transferrable, so let's say I sell you my CPO 7 today, the CPO warranty is voided.
Aftermarket warranty varies in price and service, I sell warranty that's similar to the factory one, and most of times it's cheaper than factory extended warranty. However, makes sure you can swallow the price, I ran a quote for my own car at the time of purchase, and it was over 5300.00 for 5 yr/100K!!!
2013 750i M, 2014 X1