I stayed at John's place for 2 weeks while I was waiting for my permanent apartment in London to be ready to move into. It was an affordable option and was easy for me to get to work on a bus which stops around the corner from John's house. The room itself is quite spacious, has a big window overlooking a park and a desk in case you need to work from home. There is also a private bathroom on the same floor. I did quite a bit of cooking when I was there and had access to all the cookware I needed plus a few basic groceries which John provided.
John was a really welcoming and attentive host and sorted out a fan for my room as there was a heatwave in London during my stay, which helped a lot. He was always keen to have a chat but also happy to give me my own space which I appreciated. He also gave me suggestions about what to do in the area on the weekends which was helpful.
I did not spend much time in the area as I was in the office during the week and out and about with friends on the weekends, but personally I was not a big fan of Finsbury Park itself. The house itself is on a nice quiet street, but if you walk up to the main road there were not really any nice shops, cafes or restaurants to go to and the area felt a bit unsafe at times. During the period I stayed there there were a couple of disturbances in the park next door and I would not have felt safe walking home alone at night. If you are visiting temporarily and are looking for a base to explore London I would recommend staying somewhere a bit more central with more going on, however if you're in London for work and need a place to stay it is definitely a suitable option.