I've never tried putting my cock in my ass but I have done the self-suck thing. It takes a a little work to relax your spine and loosen up but if your not too heavy or too small, you should be able to pull it off.
Once you get there, there is a downside. It doens't feel all that good. First, you're in a akward, stressed position. Second, it doesn't feel the same as having someone else go down on you. It's akin to tickling yourself.
Try it. You be the judge. | 
