The OEM X5 wheels have more positive offset than the 7er (+48F & +45R vs +24F/R), which means that the 7er wheels will stick out more (provide a wider track) when compared to the X5. Imagine adding 24mm and 21mm spacers to the front and rear, respectively, to the OEM X5 wheels...that will give you an idea what the +24 offset gets you. This may not be a bad thing...as I think the X5's OEM 19" setup could use another 20mm (40mm total) track width.

The 7er's tires are also "shorter" than the X5. If you're using those specified tire on the X5:
1. Your speedo will be off (it will indicate faster than you're actually travelling).
2. Your drivetrain (engine/tranny/diffs/t-case) will rev higher, thus wasting more fuel and cause more wear.
The tire sizes could easily be corrected by getting new tires.

So...if the hub size and bolt patterns are compatible, then go for it! But don't say we didn't warn you that tires will cause a fortune (average $1200 per set of 4). I'm only at 55K and already on my 3rd set!