I have very limited experience with it but I am not a fan. It's dirty in there, sorry, but it just is. Also, the vagina is made for the pounding of the penis - the muscles, the secretions, etc. The vagina, while it can tear during rough sex, is made to withstand it. The walls of the rectum are not made the same and can easily tear.
The eyes and the anus are the best way for Hep and HIV to enter the body.
And, no matter what, it makes me feel like I have to poop and pooping on my lover isn't something I want to do. Smells like crap too!
Now, on the other hand, I think massaging the prostate is great and am a rimming fan (for them, don't like it done to me)