I think to myself "my god...he is absolutely beautiful". I don't get this feeling so much with only physically attractive people. If it was only that I would simply acknowledge "yeah they are very nice looking".
And then I can find no other words to describe how I feel other than that, and so I would likely never actually say it to them, because what male wants to be described as "beautiful"? Maybe that matters less than I think, at least to the types of people I have been this attracted to.
I do tend to do the school girlish thing sometimes and say "he is so cute." When really I mean hot. I just feel awkward saying that for some reason.