I think it was the '97(?) 540i 6 speed that sent me deep into the M5 rabbit hole

fantastic sedans![/QUOTE
I drove a '97 for about 30 seconds when I realized how astonishingly good it was. Two years later, a '97 was my first (then a 2000, then finally a 2003). Now looking for another, doctor-owned lo-mileage one. I've probably put a total of 350,000 miles or so across the three.
They are SO good - each time I got a new one I'd start out looking for an M5 but for one reason or another never did it (the 2003 looked identical anyway). I always thought it had more engine than suspension (in something tight, like Old Topanga Canyon, at least). My BMW forum experience suggests an M-Sport requires less maintenance, too (significantly less, especially after 50K miles). The first time I took it to a track was a real eye-opener; I'd read they were good, but I really had no idea just how good. I chased down Boxsters (and sank same) all day.
And regardless that the successor was arguably a better car (whose number I'm blocking because my psyche is protecting me from its horror); it was such a design fail that I was never interested. And all that I-Drive stuff, electronic roll control, etc.... The e39 was just simply brilliant.
I also think it's really cool that you can walk into a BMW dealer and order a brand-new 5-series with a manual transmission. I can't think of another sedan that size that still offers that.