I've only been employed for about a week and I have already witnessed some of the great, and not so great, aspects of working in this field.
Flexible work schedule-Don't have to worry about saying yes to every job that comes your way
Little to no paperwork- One of the biggest complaints for full-time teachers is the amount of paperwork they have to complete. For the sub it's pretty much limited to an evaluation sheet at the end of the day.
No planning- Unless you're unlucky and have a teacher that didn't plan any lessons for the day, you don't have to lose sleep over what the kids will be doing in class the next day.
Giving back the kids- Substituting is like babysitting while going through the "I want a baby" phase. It's great to spend time with the kids, teach them a few things, but you can always give them back at the end of the day. This is especially wonderful if the kids are a handful!
No Dealing with Parents- I have had many conversations with teachers who now struggle with keeping the parents happy. Over the last decade or so, parents have become more and more likely to blame a teacher for a student's mistakes. In fact, the teachers that I have talked to that are considering leaving the profession list their top reasons for unhappiness being both the parents and administration demands for high test scores. Which leads me to another point.
No stressing over test scores- Unless you are a long-term sub (meaning a month or longer), it's really of not much importance that you pound material into the students. Of course you are expected to follow through with set lesson plans but if one or two lesson plans go awry because of you, it wont determine a pass or fail on state tests, therefore, you have nothing to worry about in that aspect.
Unreliable amount of work- If you work in a large district or even multiple districts, you may get a phone call close to every day to come in and work. If you are in a smaller community like I am, work comes a bit more sporadically. I work primarily at the middle school and am considered a "back-up" sub for 3 other schools if none of their primary subs can work. There are also multiple other subs working as a primary at the middle school that have been working far longer and have already built up a reputation. It'll take time for me to develop a relationship with the teachers and work may not even start coming regularly until next school year.
Kid's reluctance to cooperate- As a sub, you are instantly a target. From the instant that kids see you, they are going to test your limits and sometimes they manage to surpass them. It can be very frustrating, especially as a new sub that hasn't worked out all the kinks yet.
Sometimes boring- Unless a teacher plans to be gone for a long period of time, the majority of the lesson plans that are going to be left are going to consist of busy work for the kids, which means no work for the sub. Although you should still try to interact with the kids and not just sit behind the desk all day, it can still be a bit of a snooze fest.