I drove the 50i, 35d, 35i and my own naturally aspirated 4.4i the same day -
http://www.xoutpost.com/bmw-sav-foru...ives-long.html.
I noticed some hesitation/lag off the line in the 50i. It wasn't a big deal but something didn't feel right. There are a bunch of things that could be at fault if it wasn't the turboed engine - it's possible I started in 2nd (though pretty sure I was in DS), the car only had a few miles on it and those 20" wheels didn't help.
This is only from a 30 minute test drive so I'll have to take the word of folks that have driven it for longer but as always, I recommend everyone decide for themselves with their own test drives rather than taking the word of anyone on message boards.