I've heard that a few guys thought it was too much-- they were just too sensitive (wtf), but I've found it thoroughly enjoyable. I've had a couple people do it for me, with both crushed and uncrushed ice, and it feels great. Crushed can be a little different, because, if it's not crushed and the giver keeps the ice in one part of their mouth, part of their mouth can feel really hot still while the ice is cold (at least, in the beginning), and that contrast feels really good. But you don't really need to worry about the ice scraping. The edges of the ice will soften in the giver's mouth, likely before you get a chance stuff said mouth with your cock. Even if the edges haven't rounded yet, the ice is still in a wet, hot mouth-- producing even more wet; it's not like you'd be rubbing a sharp surface on dry skin. What I'm getting at is that there's no reason to be afraid of the ice cutting you or anything. I'm guessing that, at most, it'd feel similar to an experienced person lightly grazing you with their teeth-- the kind of grazing that can feel good, not hurt.
Try it. It's great.
It also has a small psychological affect for the people who like the idea of their cock being too much for someone's mouth: the ice takes up more space, which makes their mouth even more full when you get your cock in there.