I tried it once and it was ok. I want to play around with it a little more and see if i can make it a little better but what i did is
Made 2 1g starters for my brew day, back to back 10g batches. So it was 1.040 wort in a 1g growler with 1056 in one and 001in the other. No stir plate just a swirl every few hours. On brew day i decanted about 3qt from each and boiled on my stove top for 30 min and added some hops at 30 and FO.
I cant really say how much alcohol was left but the gravity went up which means some alcohol was boiled off. I would guess most of it during a 30 min boil. It was ok to drink but i need to work on the hops a bit more.
I kegged it and took one bottle to share with other home brewers, they agreed it wasnt bad and better then any off the shelf na but why do you want to do that to your beer?
To me it wasnt beer but an experiment in conservation. Like using spent grain to bake, making something out of starter beer.
If i was to do it again i would use more hops and also go straight from boil to keg to keep the o2 down.
The beer was good enough that i drank most of that gallon in one sitting to also test if it got me drunk. It just made me have to pee.