I think it depends. Copying explicitly is totally not nice but, in the end most startups(or any new idea) is just an improvement of an old idea. So, if you can

I think it depends. Copying explicitly is totally not nice but, in the end most startups(or any new idea) is just an improvement of an old idea. So, if you can make it better, you might be fine. But remember that usually you will get only a smaller portion of their business, even if you make it cheaper.