I've run into the "will call any bet for a flush draw" guys myself, and you gotta make them pay. But usually they're drawing to a better flush.

As far as betting his flush draw, I do it all the time. If I know I can call a certain bet, then I bet it first if I get a chance. Does at least three things for me.

First, if you're against others who all missed, you might win right there.

Second, it hides the fact that I'm on a flush draw. When my flush card hits, I get more callers.

Finally, it has a tendency to keep me in the pot. After I bet, say 1/3 of the pot, a lot of mediocre hands aren't going to go over the top and give me a chance to re-raise.

The drawback, of course is when the opponent has a good hand and comes over the top. You either lose your initial bet, or end up calling more than you could have. Often you end up with the pot odds to call that raise, but wouldn't have if you'd had to put it in in one chunk.