A male perspective, from the pont of view of someone who's dealt with one (only one) horizontal hood piercing, rather than the more common (I think) vertical:

It doesn't really get in the way. I'm generally not even aware of it during intercourse. On the other hand, it's kind of hard to ignore while going down on her (indeed, I'd go so far as to say it's "in your face"), but it flips up so it's no problem. If anything, it actually kind of comes in handy as a "landmark," so to speak.

I am told it's possible to knock it out, but I think you'd have to work at it.

I obviously can't speak in the first person as to what it's like to have one. It does seem (I guess) that women who get them generally keep them, so I suppose that's an indication they like them. The vertical type is more known for providing/enhancing stimulation than the horizontal. Like anything that feels good, I guess you'll wind up missing it if it goes away, so in that sense it "desensitizes." But if you worried about that, you'd never do anything that felt good, I suppose.

It's more an aesthetic issue than anything. FWIW I think the one I'm personally acquainted with is rather charming and pretty, and I'm not a big "piercing fan" as a general matter.

NizeGie is right, it won't be that much of an issue when you're having sex, though it might take some getting used to when you go down on her the first few times. It'd be terribly hard to do any damage to the clit with it, since it has no sharp points, edges, etc. And she can always take it out if it bothers her (or you).

I have one (a vertical), and I absolutely *adore* it.