Vantaggi
I have worked for this company for about three years now. The pay is better than most companies and you can reach them easily if you have an issue you need to discuss. I know I've had to switch or take-off because of an emergency a couple of times and they've always helped me out (even if they may not always be happy about it). I don't know about every site, but we usually get good people working at mine so we work as a team if someone is running late or needs to switch a day or something. If you need to take a day, just let the office know ahead of time and they'll work with you.
If a supervisor position opens up they don't take who's been there the longest, just who works the hardest and is a team player, which is fair by me.
Svantaggi
If there a couple of openings at another site, they may call you to go there which can turn your schedule upside down that week, but if you can't go just tell them and that's it.