From the recent BaT auctions, it appears we're well immersed in the 2nd big appreciation wave of E39 M5s. It's coming up on three years since I sold mine on BaT, and while I really enjoyed the M5 for 15 years and did everything I ever wanted to with it, I really haven't missed it (yet). In terms of aggressive driving enjoyment, track/time trial/autocross events, etc, the dynamics of the E90 M3 and M2C are a lot more fun than the E39 M5, but of course the M5 wasn't designed for the market niche those two cars fill
at all. If I had to go on a 500 mile trip right now, the M5 would be the choice without question. However, even with all my mods it wasn't a great agile corner carver, but it was great for a car of its weight and design purpose...if you drive it like a gentlemen's express and not a track weapon, then all is fantastic.
I'm sure I'll miss it when a car like mine is changing hands for $150k.
BMWCCA O'fest 2005 at VIR below: wonderful time, the M5 in that timeframe was a beast on track at VIR mainly due to straightaway speed capabilities.

I did win class 7A autox on the Patriot course at VIR that year against a bunch of other E39, E34 and E28 M5s. Fun times! (oh and three of us E39 M5s that were there broke rear swaybar brackets, lol)
Turn 3 on VIR main course track out:
Patriot course autocross: