I'm a retired police officer from Maryland, went for 29 years. What the others have told you is mostly true. It can be boring, tedious, frustrating, punctuated by moments of terror, and can be extremely rewarding. See my post on this thread:
http://www.early-retirement.org/forums/f29/observation-er-forum-folks-me-included-32025.html
Divorce rates are higher, no question. At a meeting the subject of divorce came up and we realized that out of 15 people at the table, 12 were married, and only one was still married to his original wife, and she worked for the PD too so she understood the environment. Not all shifts were that bad, but it takes a very understanding spouse to make the marriage work. A neighbor two doors up from me was married to a DC officer and her complaint was that "he was never home". I wanted slap her - the guy was out busting his butt to support her and the kids and she whines about his hours! What did she expect marrying a police officer?
Understand that you are going to miss a lot of Christmases, Thanksgivings, birthdays and backyard barbecues. Most of your friends will be other police officers because they're the only ones who understand what you do and why. The friends you have now will mostly drift away because of your work hours. It will take an effort to keep them. Make that effort.
There is a phase where one can become extremely cynical (I did) but that wears off for most people, but some never do. Those usually find other work.
It's extremely depressing to knock on some stranger's door and tell whoever answers that their son/daughter/wife/husband is not coming home. Ever. For most that was the worst part of it and was referred to as "drawing the short straw".
But there is action and adventure the likes of which you won't see anywhere else this side of a war zone. That can be a plus or a minus depending on how you react to it. One guy puked in the commode before every midnight shift, convinced that he was going to die that week. How he did that for years is beyond me. For me the evening and midnight shifts were when "all the neat stuff happens" and I loved it.
But I was one of the lucky ones, I never got shot or seriously injured. About 20-25% go out on permanent disability. One of the finest officers I ever worked with is 45 years old and walks with a cane. During the time I was there eight were killed, some of them good friends, one a mentor to me. You have to be able to deal with that possibility, and so does your spouse and family.
When I went into the fraud section (mostly a desk job, but interesting in it's own way) my wife was about doing cartwheels on the front lawn because I was off the street and it was straight day work with weekends and holidays off - a rare and coveted position in a 24/7 agency. My mother, then still living, breathed a huge sigh of relief.
Ever see somebody do something they shouldn't and want to do something about it? Well, you get to do that.
Yeah, if I was 22 I'd do it again, no question.