I did it for a while after I finished college and before I found a real live job in journalism. In fact, I had already accepted a job as my old high school's in school suspension supervisor when I got the word that I had a newspaper job.
Two big things I didn't like about it:
1. Early, early start to the day. I was already on that 3 p.m.-Midnight like schedule that many of us are accustomed to, so being in the school building by 7:45 a.m. was tough. I'm not ashamed to admit this, but a couple of times during the teacher's prep period, I would close the door and take a short nap.
2. It could be extremely boring. A couple of times, I had to show the same movie (it was near the end of a semester, so it was like Shrek 2 or something) for six periods. I wised up later on and started taking in piles of magazines/books or even some freelance assignments I was working on to make those kinds of days go faster.
All in all, it was a mostly good experience. I had one horror day of a bunch of seventh graders who were horribly behaved. But, talking to other teachers in the building, that was commonplace for that group. High school kids never out and out misbehaved as I recall, they were just really lazy at times. So, I didn't care too much if they just slept rather than doing some worksheet, so we got along okay.
I did once sub two kids who were severely ADHD for half a day. That included following them into the regular classroom for "inclusion". After those four hours, I was exhausted.