I did it for the SQA for many years, and also for the local council for the prelims in January. It was very enjoyable, and I felt that I was contributing something by helping to keep the exams running smoothly for the candidates, and making sure that those who needed a separate room or a reader/scribe were catered for. You need to be able to concentrate on getting the papers properly collected and packed up as well as ensuring that nobody is using a mobile phone (prohibited in the exam hall anyway) or looking at someone else's paper. We were able to sit in the hall and didn't walk up and down too much as it would have been distracting.
The pay was welcome, and a bit more when I ended up as Depute Chief Invigilator. Chief would have been a step too far for me, and anyway I stopped when DH was unwell.
Go for it, and enjoy it!