I've tried a few US agents, and like Paul, found the responses to be much faster than with UK agents and unfailingly polite. I must admit, I suspect the sheer Britishness of what I write would be enough to move my subs into the NO pile with American agents.
I suspect to get US interest for my fiction, I would need to first successfully publish a book in the UK before it was considered to be adapted/translated for the North American market.