I think that Yao Mulan is too perfect (she's basically a Zi Wei-type character in the 1900s), but I really liked Vicki's performance. Some people say she didn't have that gentleness and that she was like a fish out of water, but Mulan was also supposed to be really smart, and even more dependable than men, and Vicki really gave Mulan a backbone. The scene where she cries when Sunya complains about their marriage broke my heart.
The second son's wife (Suyun) is a real brat who goes to work for the Japanese afterwards, but the actress was pretty good. However, in Ban Sheng Yuan she played a similar character (spoiled rich girl), so I think if she really wants to develop her acting, she needs to take on other roles. That's why I really respect Vicki's decision to do MIP - it's a different, new challenge for her.
Mochou (Mulan's sister) was way better in the book, IMO. In the book she was more womanly and practical, but in this she was just spoiled and unreasonable and noisy.
Sunya's monotone sometimes, but it's more displaying his nonchalant attitude. When he erupts he does a good job.
For those who want to know a spoiler, the really really annoying and bad character in this series has to be Niu Huai-yu. He was okay when he was with Mochou, but in the end he was more evil than his sister Suyun (who at least had the brains to change into a better person). Meanwhile, Niu Huai-yu is off helping the Japanese, acting like he's Japanese, and totally having no concern about his home country, taking troops in to ruin the Zeng house, killing Manni and her son...he's just...ugh. Anyway, still have two more episodes to go.