It's an extremely sensitive topic. As much as I feel you said something great, people will feel insecure about their stutter. Given that it causes them myriad problems. Is it a fascination for you, like you hardly hear somebody stutter, or you genuinely like to hear somebody stutter? You'd have to let them know in a non-judgemental why you're interested in it. Of course you'll always be faced with mixed emotions from the other person.