I have ESS for couple of months now. In general, pretty happy with it. Installation is easy but you need to take a backup of the OEM software
every time car comes back from the dealer. I learned that the hard way...
ESS stage 2 is supposed to give you 360hp (for some reason I had 390hp in my mind
) and 355 ftlbs torque, which are impressive assuming they are being conservative to protect the car in the long run.
The fact that ESS is going to be a very expensive paperweight when I sell my car was a major concern but had to deal with it. They also have a very responsive customer service, which is very important for me.
I have not tried COBB but I have also seen Active Autowerke's 1M tune applied to 35i which was quite impressive. The car also sounded better...