Maybe I'm just a noob but I thought the M2 was legit.
It's as pure a "BMW" experience as you'll get. I was enamored and it moved to the top of my shopping list.
I didn't drive the M5. Experiencing the power would've been cool but I just had a 540 loaner that kind of bored me so I didn't think the M version would be that thrilling. Maybe I was mistaken.