(Toronto, Ontario, Canada)
I am a native Chinese speaker, who have came to Canada during my teenage years and learned English, French, Japanese, Korean, and some Spanish. The most difficult language that I`ve learned in my life? Still has to go to Chinese, by a large margin. So here I'll rant a bit about my experience and hopefully provide some tips for people wanting or currently learning Chinese (Mandarin).
Written System: Like many have said, the Chinese written system either in simplified form or (even more so) traditional form, is indeed a monster. In fact, I learned Japanese in 2 weeks --- I practically only needed to learn the pronounciation, because the written system in Japanese (often referred to as the harder part of learning Japanese) rely heavily on Chinese characters. Even for native Chinese speakers, written Chinese come slow and goes fast, that's why you see many Chinese immigrant losing their ability to write Chinese after just a few years abroad. And academics in the Chinese Language literally spend their life time constantly learning new characters and reinforcing what they've already known. I'd advise constant practice on writing in Chinese for all who are learning, just reading won't do --- Simply recognizing a picture doesn't mean you can paint it, many of us have fallen into that trap and by the time we realize it, we can't even write up a full sentence without resorting to a dictionary. Also, a learning system regarding the written system of Chinese is also available (Pian Pang Bu Shou), one should really look into it if you'd like to eventually not rely on Pinyin for your communication in Chinese.
Grammar: Grammar in Chinese is different and a monster of its own if one is drafting official documents, but fortunately it CAN be a LOT more flexible than any other languages that I've come upon. Meaning, you can make a sentence "make sense" in many ways without others frowning on you. In fact, the fluidity in Chinese grammar can sometimes become a problem of its own especially if you'd like to get a taste of classical OR modern
Chinese Literature (in my opinion, though troublesome, this is the single most wonderful thing that you can get from learning Chinese. The beauty of Chinese Literature is so vastly different from any other language, you simply can't get it anywhere else. And there's 5000 years of supplies for your appreciation too.). Almost all character can be a noun, part of a noun, a verb, part of a verb, an adjective, part of an adjective etc depending on the sentence. If you'd like to understand classical Chinese --- a requirement for all Chinese students past middle school but fortunately not critical for business/conversation --- The difficulty and requirement in the understanding of Chinese Grammar increases dramatically.
Pronounciation: I had to learn Mandarin when I started going to elementary school, because I spoke a local dialect (Cantonese is a dialect, Shanghainess another, Fokien another, etc) before that. Different from British vs American accent, many of these dialects are so different that speakers of one can understand nothing from another. Because of that, not all Mandarin sound the same, native speakers of a dialect often carry their own accents --- Fortunately, this means that people in China most likely can still understand you even if you spoke in a noticeably poor accent, because they've had to deal with so many different accents in their daily lives anyways. Getting the Mandarin accent "right" is quite challenging, especially since there are so many different accents (many consider the Beijing accent to be "proper", however just as many consider the Xi'an accent to be "proper", you can't please everyone). Therefore, I recommend that when first learning Chinese, try to not focus on the accent TOO much, once you have the basic grammar rules and the important vocabulary memorized you can go back and fix these accents. Like mentioned in other places, pinyin is a must-learn to efficiently learn Chinese, in fact it's a good idea to start with pinyin above anything else such that one can quickly learn conversational Chinese bypassing the complicated written system.
I hope this has provided some insights into the Chinese language for you!