It all depends on the woman, and to what extent its taken. Its a turn on to be appreciated for what you have, to know that they are looking at you as a source of something they desire, however...
There is also a point where some women take it to a point of almost too much persistance. Its cool to talk with them about it, cool to do things about it, but when you are making a comment that is unrelated, and every single answer to everything has some innuendo or reference to size, it can get old after a bit. If you are looking to them for the sizequeen aspect alone, this can get overlooked, and at times is just what you want.
Overall, it just depends on what you are looking to get out of your time with the other person. If its a pure sexual thing, then cool, if you want more, and they aren't willing to offer more than size worship, thats when you can have issues.
Its not the sizequeen aspect thats a turn off, hell I would bet a good percentage of people here enjoy the fact there are. Its not always the easiest thing to talk to someone about issues relating to size, and who better to discuss with than someone who knows a lot about it. Its just how they approach it, and handle themselves.
As always, my own opinions