The hours are horrible. Don't be surprised if there are weeks where you work less than 5 hours, even if you've been there a few months. Scheduling can be difficult since only one week is posted at a time. If I work Friday, I will not know until two days beforehand. This might be different at AMC Theaters in larger cities. My location is clearly overstaffed and disorganized.
Training was awful as well. No one was in charge of this, or held accountable. My training was all done with co-workers, some who were hired three months before me. I would not suggest this as a first job because of this. There was a lot of little important details I was never told until I was scolded by a manager. It is also easy to forget some things since it can be several weeks between working different shifts in concession and ushering.
Another major downside is management expects you to always do busy work. I don't mind this that much considering I've lived on a farm, but it can become too extreme. For example, I was asking one of my co-workers some questions about using the cash register when it was down-time. I assume the manager was watching us from the cameras. The manager came out of the office twice and immediately harassed us asking why we weren't cleaning. Adults, don't be surprised if you are treated and viewed the same way as teenagers. If you are new, they will automatically assume you are lazy regardless of what is on your resume.