I have the Aneros Progasm and really like it...would recommend it.
It's easy to manually stimulate the prostate with that one, and also to passively stimulate it by flexing and releasing your PC muscles. Humping a pillow works, too, kinda like being fucked while fucking.
The Progasm is much better than the smaller models, and you can definitely do some serious milking of the prostate with it. Read some testimonials on the site by guys saying it was almost too large to insert...but please, give me a break...if you're gonna insert something in your ass you want to
feel it, right?
The automatic tense/release reaction you get during an orgasm causes it to rub against your prostate when you cum. Great orgasms. The longer you edge, the more you milk and the more amazing the orgasm is.
I haven't had a hands-free orgasm with it yet, but that's only because I can't resist bringing myself off manually in conjunction with the deep anal sensations.
Feels DAMN good for my money.
See this hands-free, slo-mo cum vid with an aneros (not me).
http://www.empflix.com/view.php?id=29478
Notice the white thing below his balls...that's the part of the aneros that stimulates the base of your dick as you flex.
Here's another with the same guy, only you can't see the Aneros on this one.
http://www.empflix.com/view.php?id=29150
Enjoy!