First, let me qualify my answer. I'm a science fiction and erotic horror writer, not a doctor. My answer is my opinion based on my empirical experiences. Take it for what its worth. Male Positives: The tight feeling feels great! Female Positives: Your boyfriend will love you for it and may agree to extra cuddling or other stuff that's ordinarily difficult to get from him. Male Negatives: A slight risk of infection from e-coli that normally inhabits a person's bowels. Because her rectum can bleed easily from the abrasion, you can get her blood on you. Contact with another person's blood carries a risk of STD. The spinchter is so tight that the struggle to get it in may cause ejaculation before penetration is very deep. Female negatives. It's extremely painful for most women. Using K-Y Jelly helps a little. Because another person is depositing their body fluid into your body, there's a risk of STD. (Okay, I stand corrected. As JollyMommy says below, there are a few women who do enjoy it. Unfortunately for us guys, such women are way in the minority :-(

Ok male positives: Most guys seem to really want to get a girl that way. It is like a conquest I think. Tightness Some like the "rear view" of the girl Less chance of pregnancy Male negatives: Lots of foreplay where he does the work the girl gets to lay there (to do it right) Chance of crap on dick. Not likely if girl does clean up before hand Chance of infection (condom helps) Chance of blood contact (I'd say equal with vagina personally) Female positives: If done right the orgasm is amazing and very different from vaginal orgasms. For me much preferred. Very strong mind influence. The taboo side, the being taken from behind idea, all that works to make it very sensual. I get to lay there while he does most of the work during foreplay and just enjoy Female negatives: Some pain. Especially if guy is new to it and doesn't know what to do and not to do. Lots of lube needed. Gets on things Chance of "passing gas" and embarrassing myself. Most of the time it's not me... it's trapped air but all that action down there has sometimes jarred gas loose and it comes out! When the cum comes out it can feel weird and slimey. Worry about what the guy thinks of you since you let him up your most taboo place.

I don't fully agree with a lot of what Jennifer Jason said. Read this post. It tells you what you need to know about how to do anal sex the right way: http://mflife.wordpress.com/2006/10/12/anal-sex-101/

Positive for both: It feels good to do something naughty and primal.

Negatives are the bug and sometimes pain Positive are totally enjoyable

Positive....good sex/orgasms. Negative....pain/shit on your dick. If done correctly it can be a good change up from regular intercourse. If not, it can be a painful thing that the woman does not want to repeat. Then again some people like pain during sex.