The best thing is taking that hot load that I earned, the worst is when it's over and having to wait for my next one

The best thing is earning that load or more loads from several Tops, nothing beats getting filled with spunk, the worst thing is when a dirty fucker gives you a STD that you're not waiting for.

Agreeing with what everyone else said, feeling cum flooding in your guts as being the best thing, and as far as the worst, when I have a guy that visits me without giving me enough time to prepare myself.

I am aroused knowing I am willingly becoming a receptical for another man to inject with his cum and I want to feel that warm slickness as I am being bred. Also is great for group sessions to lubricate that next entry. Nothing bad. 

Best thing is that the sex proceeds naturally without interruption.  

The sound a top makes when he's shooting his load as he puts it in as deep as he can

BEST: Feeling his cum explode inside me.

WORST: A lot of men/guys don't know HOW to fuck. I'm not "just a hole" (not that there's anything wrong with that!) but observant eyes etc can determine if it's pleasurable or painful.

Best part of being barebacked and bred is the feeling of having a man deep inside, I love the intense feeling and frequently ask for harder and deeper.

 

Worst is spending a fair amount of time setting up a meet and they don't show and don't bother letting you know they aren't coming. They have no idea of the prep a bottom goes through so that they enjoy being inside you.