Wife and I use the non-vibrating version of this (the original FeelDoe) and like it quite a bit. It penetrates me fairly deep and in the right positions she can ram it hard into my body in such a way to cause minor discomfort -- she likes to see me wince and my breath catch. I don't mind one bit. She has been known to use this device solo on herself (it cleans easily and we use condoms on toys).
This will loosen a guy up quick, making you wish it was slightly larger. We just saw that there is a new "Stout" version that is our cart right now. Will update when it arrives.
FWIW, She's always worn a 4-strap harness when using the FeelDoe. We've never tried it 'solo' the way other commenters have noted, probably because we always assumed you need the harness. It would certainly save time...