Pros
Every day is different. This is especially true if you work at multiple sites or you work at sites where you come across hundreds if not thousands of people in a day. You will have some of the most interesting conversations with people that you will most likely never see again in your life. You will learn things about Humanity that people will not believe when you share the stories. Security can be very interesting work. Remember, you get out of this job what you put into this job. If you are motivated and you have heart you will working in the security industry.
Cons
Long Hours. At most sites it is expressly written that you can't leave until you are properly relieved. This means that you may be close to finishing an 8 hour shift when you receive a phone call from your supervisor or manager stating that your relief has just quit and they're working on finding a replacement. Consequently, you have to hold over until relieved. In California, if you are working 8 hour shifts you can be held over for an additional 4 hours. Hopefully someone will relieve you before you reach that point. The longest shift that I've ever worked was when I volunteered to work a double after somebody called in sick at the end of my first shift. It didn't occur to me that was the night of the “fall back” change in daylight savings time until the time change happened at 3am. There's nothing like working a 17 hour shift.