Pros
If you are interested in being a teacher or just need a part time job this can be an ok job. You don't really have a supervisor and you can create your own schedule. If you need time off to run errands or go on vacation you just put in a non work day. No begging the boss, no fighting with the employees that have seniority, no BS. You want a day off you have it. You also get all the same holidays the kids do and if you are lucky to get a great school, the kids are awesome.
Cons
For every one great school in HISD there are like 5 terrible ones. It is very hard to find an assignment at a school like Carnegie Vanguard, Debakey, HSPVA or other blue ribbon schools. When you go to a normal or title one school the kids can be trying at best and a straight up zoo at worst. The disrespect the permanent teachers get alone is maddening so you coming in as a sub you might as well just have fresh meat tattooed on your head. A lot of schools you can call the office or an AP and they will try to help you but there are a few where you are truly on your own. Another con can be the pay. If this is your only source of income it is not enough to rent an apartment even if you work 40 hours a week. Many employees are subbing as a part time job or are retired teachers getting extra income and something to do.