Unfortunately, if you are going to be pegging a man with it, you will absolutely need a harness. It's like a tug-of-war your vagina simply cannot win against his anus, and even if it goes well at first, the additional G-spot stimulation from the bulb inside you will eventually make you too wet to hold it, and at that point the bulb will slip out, possibly painfully (it was for me ponce or twice), and then you'll wish you had used a harness to begin with.
I've not yet had the experience of using it on a woman, which is how it was intended to be used, but the muscles of the vagina are much, much weaker than those of the anus and sphincters. After all, the anus was meant to keep things in, but the vagina was designed to push things out.
That's not to say I don't entirely recommend the Feeldoe for a beginner's strapon experience, though if you're familiar with strapon sex and want to get rid of your straps AND fuck a man in the ass, your best bet is probably the FunFactory Share
. The bulb that is held by the "giver" is much more ergonomically designed, as well as just plain bigger, and the finish of the silicone is matte rather than glossy like the Feeldoe, which makes for better hols all around. But if you're looking to use it on a woman, I've heard most women have no problem using it that way.
Hope we have been helpful so far
Tim - It looks like they don't carry the Sysil Edge anymore! Too bad, I was hoping to get one someday.