I've been using Cobb with my Z4 for a little bit. Right now it's off since I just got the newest software from BMW when I got my roof serviced. Cobb is still working on the new firmware for the ROM.
But my experience with Cobb on the V4 maps was pretty awesome. I had the opportunity to drive a good 4 hours right after installing it with my downpipes and exhaust. The car really pulled a lot harder... With the Cobb off the car right now, I really don't enjoy the stock power of the Z lol.