Start/Stop is fine. I have an MY15 KL which has it and initially, I hated it. I would turn it off at the beginning of every drive cycle. But after about 6 months, I thought, I really should give it a go - give it the benefit of the doubt if you will, so I left it on and am quite used to it now (16 months of ownership).
You can manage it quite successfully by 'feathering' the accelerator (if you choose not to hit the button in the centre stack to turn it off).
When you do allow it to do its thing, it really is a sub-second restart - maybe .8 of a second (?) - my 12 year son timed it one day.
You get used to it. It is as unobtrusive as I would imagine they could possibly make it. It's really no big deal.
We've decided to replace the MY14 GC with the '17 GC whenever that arrives in Aust and I'm quite looking forward to comparing the feature between the Grand and the KL. If it works exactly like the KL, then I will be happy.