Got my letter from Seat to get the software update and I chose to ignore it. A couple of weeks later I got another letter saying that I hadn't booked the car in yet. 
My car is a 2012 Sport Tech TDi 140 and I didn't know about any "opt out" so booked the car in. The job took an hour and a half but some of that time was spent washing my already clean car (I'd washed and hoovered it out that morning). They also completed an obligatory health check I hadn't asked for. After failing to find any faults they left it outside to dry streaky in the sun.
As a token gesture,they presented me with a Seat holdall bag complete with Seat keyring, Seat drink bottle and some sort of towel sized Seat cloth for being "a valued customer".
It's been two weeks now and I can honestly say there's absolutely no discernible difference in either performance or economy.
That's my experience so far and hope it gives an element of confidence that the job is nothing to fear.

My car is a 2012 Sport Tech TDi 140 and I didn't know about any "opt out" so booked the car in. The job took an hour and a half but some of that time was spent washing my already clean car (I'd washed and hoovered it out that morning). They also completed an obligatory health check I hadn't asked for. After failing to find any faults they left it outside to dry streaky in the sun.
As a token gesture,they presented me with a Seat holdall bag complete with Seat keyring, Seat drink bottle and some sort of towel sized Seat cloth for being "a valued customer".

It's been two weeks now and I can honestly say there's absolutely no discernible difference in either performance or economy.
That's my experience so far and hope it gives an element of confidence that the job is nothing to fear.