I have had four suits adjusted by John and he is currently working on two more. He is an excellent tailor. He takes time to note every detail and points out areas that he feels should be worked on, unlike some tailors who won't point anything out and try to do the bare minimum to just get the job done and get paid. All of the suits John has adjusted for me so far fit perfectly and look great.
His quality of workmanship is outstanding; when you get your suit back it will look and feel like it was custom made for you. Literally every detail is attended to; inside and out your suit will look like it did off the rack. That means no shoddy stitching or asymmetrical work, no hastily rushed seams or or hems that will weaken in a short period of time, everything will be 100% masterfully done.
Having said all that, be advised that you get what you pay for, and John is not a cheap tailor. Each of my suits cost $145 to adjust (except one which was $130 because the pants only needed to be hemmed, no waist or seat adjustments.) If you want to get your suits tailored economically, you can get it done cheaper elsewhere, but if you want a perfect fit and high-quality workmanship, you are going to have to pay for it.
In my opinion John is on par with the custom tailors in Manhattan.