man I wish THAT M5 engine was in my M5 --- I'm so jealous you got to drive it, cuz I've been wanting to. I love the V-10 powerplant, but I'm not much for the new 5 series design (seems that with the newer body styles, you either love'em or hate 'em). Those things are something like 500HP now so I'll bet it was fast! Wait until you take something like that down the deckers run ... those things are on rails.
Then of course here's the mods ....
www.dinanbmw.com is the *only* way to go with those.