I drive a 2017 VW Polo 1.0L. I use the car for primarily short journeys with the occasional motorway run. The car has a start stop function but I am skeptical of the benefits.
Will it be a useful feature for somebody who does mostly short journeys (under half hour)?
Generally, these systems don't activate until the engine is warm, so you probably won't see it for the first 5-10 minutes of your journey - of course it largely depends on the nature of your journey as to whether that's useful or not, plus, as Solar Mike says, what you consider useful.
For example, if your half-hour journey is 20 minutes pottering through the suburbs, then 10 minutes sitting in traffic as you approach your workplace, then it will probably be useful as it will cut in as you sit in the traffic, helping to reduce your emissions (and fuel cost!)
When I had about 60 000 km on my 2011 Toyota Yaris with start/stop, the start/stop meter showed the engine had been stopped for 14 hours. So, 300 000 km, a useful lifetime of a car, means the engine will be off for 70 hours. At 0.7 l / hour, this is 49 liters of saved fuel. Depending on the fuel costs, the expense may vary, but it'll probably be below 100 USD. A new battery costs 50 USD, a new starter motor far more than that.
Based on this, I would turn off the start/stop system if there's the possibility to turn it permanently off. The extra wear on precious components (battery, starter) more than outweighs the benefits obtained. Sadly, my 2011 Toyota Yaris had the possibility to turn it off only for the current trip and it would be automatically turned on for the next trip.
Also, consider this: most start/stop cars cannot use the heater or AC when the engine is stopped.
My current car, a 2016 Toyota RAV4 hybrid, also has the capability to stop the engine and in this case it is very useful as it can even turn off the engine when in motion, powering the car only from the battery. Hybrids are in a league of their own. Both heating and AC work when the engine is stopped due to electric pumps and compressors.
