It's OK. I wasn't impressed with the E39 at first either. It looked out-dated from day 1. The rear quarter panel has much to do with it. The E60 definitely is not spectacular, but I admit that it looks tougher on the outside. I bet if you parked one next to an E39 M5, you can definitely notice the age on the E39, as shown on this board. Interior wise, the dash is "Cadillac"; looks like an old geiser car. Basically, Bangle's design is terrible, inside and out. Nonetheless, it's still a more fresh look than the E39. I would be more tempted to shop for an MB the next time around. I like their new look. If their new 6.2L engine turns out to be good, then the decision should be simple. To reiterate, the E60 M6 is OK.
I forgot, the E60 needs a better wheel design and they need a metallic blue color option like LMB or avus.