I found Duolingo to be too slow paced, and not effective enough if you are serious about learning a language.
The way Duolingo is structured, it will take a week or two to just get completely past the "Hello, how are you, my name is X" phase
Eh, Duolingo is alright if you are learning a language ab initio, and the linguistic distance between the source language and target language is not too far.
Full disclosure: I do not recall having tried en↔zh courses. And in fact, I have not used Duolingo for a long time, so your mileage may vary. And for the one language (en→nl) that I was seriously(?) using it for, I never got around to finish it.
Duolingo is really slow-paced precisely because the course writer had to assume that the learner knows absolutely nothing about the target language. This is probably why the first few rounds are rehash of the same handful of words: to hopefully hammer down a solid foundation that the rest of the course can build upon.
Duolingo is unfortunately really light on grammar, even at times when it would be very helpful. Without helpful people in the comment section, one is stuck with the impression that the language is completely arbitrary (… which I guess it is? But still). At least the comment section can usually help people look up proper explaination, which is something, I guess.
Now, as I've said, I do not remember having tried en ↔zh courses. However, I have tried en↔ja courses (in both direction), having learned at least basic Japanese elsewhere, and I was…
less than impressed. Duolingo model of mapping vocabulary and grammar from source language and target language and vice versa and throw arbitrary sentences at people really falls apart here. Without prior knowledge, one could walk away from the first few lessons thinking that grammatical particles are somehow verbs. It would probably be very difficult to unlearn. This sort of thing may or may not happen with Chinese courses, but given the linguistic distance between en and zh aren't that close, I am not really confident about Duolingo's model of language learning in this case. But then again, en and zh are both SVO, unlike ja, which is SOV, so I guess it shouldn't be too much of a problem.
So yeah, it's probably okay for very beginning, but I would not recommend, at least not as a first resort.
曰? You sure that's what it's called? XD