Who me? I wrote a write up on my opinion of my M3 vs. my dad's Legend Coupe:
http://www.acura-legend.com/vbulletin/showthread.php?t=108018
On the question of what people upgrade to? If you're considering BMWs, I'd say look toward the e46 BMWs: my E36 M3 is pretty much the polar opposite of my Legend. At least in the e46, they tried to build a decent interior (and didn't do half bad).
I've driven the G35 Coupe and frankly liked the BMW coupes much more but the G35 is a nice car. The G35 felt a little larger and a little more plush but the BMW manual transmission was far superior.
I totally agree with B+O though: the Legend was an enigma, especially the coupe and even moreso the 6-speeds. No car on the road hits the balance Acura hit with these cars. The only reason it was easy for me to move on is because my M3 feels so different from my Legend in all the areas my Legend bothered me: it has plenty more power, it handles worlds better, and it's a manual. If it hadn't been so hard for me the find what I wanted in a Legend Coupe 6MT, I probably wouldn't have the M3.