As someone who has learned Chinese as an adult, I would recommend against it unless you have the opportunity to do so without sacrificing considerable opportunity costs or have the luxury of not having to worry about opportunity costs. The learning process is considerably more time-consuming and challenging than a European language, and you cannot learn it to a functional level from taking classes. (There are many foreigners I've met in China who took four years of Chinese as an undergraduate and were astonished to discover when they set foot in the country that they were totally non-functional.) You have to actually live in a Chinese-speaking country, and it's very hard to get a decent job in China unless you're moved there by a multinational and retain your salary and benefits from the home country. Even then, if you're working a regular job, you simply won't have time to learn the language in a reaonable tme frame. I know plenty of expats in China who have been working here for 7-10 years and still can barely ask for directions in Chinese.
Finally, if you think you can simply show up in China and people will be beating down your door to give you a great job, think again. The idea that China is full of potential is a total myth for Westerners. There are almost no opportunities for Westerners outside of teaching English or other jobs unrelated to professional technical positions, and no Chinese-owned firm I've heard of has ever given a Westerner a management position with any authority. Whereas in the United States, being a non-U.S. citizen does not impose a glass ceiling, in China quite the opposite is true. You simply won't make money here unless you are working for a multinational and are moved here from your home country rather than someone who moves here and is then hired in-country, in which case your living here is taken as a clear signal you're willing to work for local wages.
In short, people who talk about Chinese as a way to open doors and create opportunities are simply out of touch with the realities on the ground in China.