The first few times it can be sorta painful if the top doesn't use the right about of lube, and there isn't foreplay, I remember with my first bf back in 04 he wasn't so good at the foreplay and I would lay on my stomach squizing his favorite bear (now how gay is that) about to die in pain, I"m so glad to have found better tops now, and that it is definatly more pleasurable than that.
If I'm trying to take something real big I find that just a little bit of alcohol in the system can help, not too much though as then I can't cum, and I don't enjoy the sensation as much, but just a bit can help me relax.
Overall bottoming can be a great experience once you get use to it and you can try all kinds of position.
My take on positions: If the guy is really big I like to be on my stomach to start, then flip to be on my back, and sometimes it can be hot to top the top that way the bottom control's the insertion. Its all just a matter of discovering what you like, but if your already an experienced top you should have no problem discovering what you like as a bottom. The key is patience don't try to take it all in one take, LUBE LUBE LUBE, foreplay (love when a top knows that he's going to have to loosen me up first without having to be told), recently I took a BIG cock 8+ and thick, and it was no problem at all which it could have been since I hadn't taken a cock since like June/July of last year. He was able to go balls deep it was AWESOME, it was hot feeling his low hangers slap me between my legs, he even went so far as to fist me a bit (hadn't ever had that done before, I didnt quite realize what he was doing until he had like a good 3/4ths of his fist inside me, Have learned that I like SILICON lube alot better for anal that water based as it stays alot lubier and you don't have to constantly reapply and it doesn't get all sticky. I so need to order more.