I had mine from 2009-2015, drove it 29k miles and sold it to a friend for $5k+ more than I paid (it was in much better shape than when I bought it though). While I absolutely loved the E39 M5 and it was amazing for the time (and still looks wonderful), in comparison to the 5BW:
- Engine has a similar powerband and response, but sound is much more muted (and I never grew up loving American V8s, but the Blackwing sound is another league) and the power is obviously far less. Road & Track compared the two and said the Blackwing pulls harder in 5th than the M5 does in 3rd. I did the math myself and it's true! Pretty crazy to think about given the M5 had a ton of torque for the day (369lb/ft @ 3,800rpm).
- Shifter is far less solid and more vague even after some aftermarket mods. Clutch is notoriously weak - a popular and expensive upgrade was a larger clutch adapted from the V12 8-series.
- Handling had a lot more body movement, vagueness and understeer. Steering was recirculating ball and lacked feel.
- Brakes were single-piston and it showed. I actually autocrossed mine a few times and even with high-temp fluid would get fade.
- Ride was quite comfy (non-adjustable suspension), but from my memory the Blackwing in softest setting is even more compliant. Only exception is low-moderate speed in-city driving where Magneride can't hide the stiff swaybars over one-wheel bumps like manhole covers.
The BW rear seat is also much roomier except for headroom. The M5's wasn't much bigger than my E90 M3's. Gas mileage is the same as 5BW.
Again, amazing car for the time and still holds a real soft spot with me - a modern classic no doubt. But GM had the advantage of almost 25 years of progress, and it shows - it's simply much better in all dynamic categories. Also should mention the E39 was by far the least reliable car I've owned. More problems than the 6 other cars I've owned combined.