I ran it for about 6 months, got blood work done and my thyroid was normal. That said, I've always run it relatively low - 25mcg.
Based on my own experience, I would disagree that 25mcg will "do nothing for you". We are all different. 25mcg, for me at least, keeps me from gaining fat if I'm eating more. If I'm dieting, it keeps Thyroid hormone constant for the whole diet, rather than crashing off.