I originally ordered an xDrive50i and, while waiting for that vehicle to be delivered from Spartanburg, decided to upgrade to the X5M. I debated it quite a bit during that month, trying to weigh the value of what I was getting for my extra $30K - for such a premium as this, would it be worth it? What do you get for that? The obvious answer is a performance edge - a half second off the 0-60 time, higher (limited) top speed, better handling (.96g skidpad vs. .81) and stronger braking (152 ft 70-0 vs 172 ft 70-0). The xdrive50i performs like you would expect a nice luxury SUV to perform, while the X5M performs like a supercar. Is this meaningful though in day-day operation, especially over the M Package xdrive50i? Probably not, if I'm being honest - no way you can tap the full potential of the X5M on the streets. If you're being halfway sane, at least. Oh, and with it's quad exhaust, the X5M sounds much nicer too - a deep roaring baritone exhaust note when you lay it down.
Outside of performance, the X5M has a meaner look from the outside - sitting lower to the ground than the xdrive50i. I love the wheels and the spoiler which add to it's aggressive styling. And it is available in Doninngton Gray, which is (I think) one of the nicest finishes I've ever seen. There is M badging everywhere, which doesn't mean anything to me (or most people). But, if you're into status, whatever. I like how understated the X5M is - its aggressive exterior non-withstanding, it doesn't look like it should be as fast as it really is.
On the inside, the only real difference I can see (besides further M badging) is the amazing front seats - I was coming into the X5M from an older 7 series. The xdrive50i had seats much like my old 7 does. The X5M has nice side bolstering that keeps you confidently in place. Some reviews on the seats were mixed - I find them to be firm, but really, really comfortable. I read reviews on the ride quality as well, given the stiff suspension - I find it rides better than my 7 did (although I went with the 20" wheels vs. the 21" - I think the extra rubber helps).
There are a few downsides (besides price) - full efficiency is terrible. I'm less concerned about cost as I am the fact I find myself at the gas station every 4-5 days. I also think some of the controls and the iDrive could be more intuitive. Nice to have all the customization options, but challenging to get used to. Also, given it's heft and high ride height, it doesn't deliver that "rides on rails" experience I got with my 7 series. There is a hint of lean in corners and although it sticks really well, I don't (yet) have the confidence to hit the curves like I would like to. I like that you can customize the lift-gate opening height, but am wishing it could auto-sense and stop if it finds an obstruction from above. It's a nit, but I find that if I have it set to either of the two highest settings, it actually makes contact with the ceiling of the parking garage. If I have it at the third highest setting, it doesn't, but then offers somewhat limited access. So I have to manually adjust based on the setting for ideal operation, when I feel this could be automated.
Finally (and this is more of a BMW thing than an X5M specific thing), I found myself at the dealer being pushed on a bunch of additional protection plans (wheels and tires, windshield, front end protection, wear and tear insurance, extended warranty, extended maintenance, lo-jack). I ended up paying $18K extra. I would have liked more time to explore these options than I was given. I asked it I could add them within the first 30-90 days and was told no - you must do it at time of purchase. I said yes, before really getting into the details on what was afforded there. That's a lot of money to spend on the spot. I'm glad I did it though, given this is the first really (really) nice vehicle I've owned and want to keep it perfect for as long as I can.
I have now been driving the X5M for close on three years and have grown used to its foibles. The performance and reliability are excellent but the software and related technical aspects are a pain and I defy anyone to remember how to use all the different nick nacks. If I have a gripe, it is the suspension which, although it is better than the previous model - it could not have been worse - is still substantially worse than the Porsche Cayenne Turbo with which it competes. They have tried to substitute big rubber for suspension sophistication and it does not work. There is also no ride height adjustment which the Porsche has and this seriously limits the car's off road ability. It is a good car on tar make no mistake, just not a great one and off-road it is a joke. In a nutshell, in my humble opinion, it is not worth the price and I will not buy another one.