It is good to be inspired by the examples of others. It is self-destructive to ever feel negative towards someone because he has something you lack. I was amazed the first time I heard someone joke that he hated someone who was real good at something.
Those who achieve the most don't waste time with this negative emotion. Those who do feel it are almost certainly failures and underachievers.
Let's choose a mundane example, money. I'm always glad to hear of people in my profession who make more money than I do. This is great, it means that I haven't peaked out yet.
Another good example is martial art studies. I want to attend a martial arts school where I'm the LEAST SKILLED student in the school, not a school where I'm the most advanced student. This is a totally practical and selfish attitude which will result in my being a better martial artist.
In case you're wondering, I'd define positive envy as admiring what someone else has achieved, and wanting to also achieve it yourself.