the cadillac may be a good car, but it certainly is a brute. before i get into my M5 i always start to smile of joy when looking at it, outside and inside.
the M5 is extremely capable, just not the most involving drive. if you want the latter (and i am definitely in that segment, at least as a second drive) then just get a real sports car. ferrari 458, lambo gallardo, porsche 911 (GT3 at best) or the likes but not a cadillac, for god's sake!!!