Nictrix said:
Only downside that I can think of is the cost of tyres that most seem to say need replaced as a full set and will cost around £1k.
Another reason to go for 19's (285/45 & 255/50). I've just bought 4 brand new Goodyear F1 * rates run flats for £550 from Costco. My wheels are off the new F15 X5.
I don't think they look undersized if you look at my photos above. Big sidewalls are great, not just for comfort but I use it off road (nothing serious) now and again so don't need to worry about damaging the wheels.
Replacement will probably be the new X3 model in a few years, looks fantastic, just need them to come down in price.