I'd say pretty popular for not being American! My coach loves him and everyone at the tennis center always asks me where I got my RF hat.
His practice sessions at Cincy were packed. Also, I went the the USA vs. Spain Davis Cup QFs last year for all three days and I saw a ton of people wearing RF hats.!
Naturally they're not gonna root for Roger if he's playing against Roddick, but I'll never forget the way the crowd really pulled for Roger @ the US Open 2008. Especially during the Andreev match. And they played 'Still the one' after all his matches, except after he won they played the Darth Vader theme. Also, Roger said on the morning shows "I'm starting to feel a bit like a New Yorker now."