My dad had one on his F2a ( a 35mm f/2.0) and sold it for the 35mm F/2.8. Apparently it was not very good and it was way too soft. He said his 35 F/2.8 blew it out of the water. It is remotely possible he had a bad copy, but I kinda doubt that.
Ken Rockwell posted daisies about this lens, so it has to be incredible, right?
BTW - I was tempted to by the Sigma HSM 30mm f/1.4, I absolutely hate it: it cost more than $500 and is useless at f/1.4 due to autofocus errors (to add insult to injury, it's also physically large and heavy...)