I have been using them in China mainland for 2 years.
Base services are OK (computing, object storage, load balancers), CDN is actually quite good.
They are rolling out new services very quickly but the quality is still far from the AWS standards and feature rollout is slow outside of China (for instance, no way to replicate a bucket from Europe to Hong Kong).
Support is cheap, quite reactive but not very helpful.
ruby SDK is not very helpful (wasn't translated in English until recently, last upgrade removed the object storage SDK from the unified CLI to a dedicate one which is still in beta...).
So it cannot really compare with AWS globally, but for China mainland it might be the best provider. And they are moving very fast.
At my work we use them to host a static site and to serve streaming video content in China. We've also used the elastic compute service before. Their platform is surprisingly comprehensive and similar to AWS/Azure but is not perfect. Some parts of the interface do not have an English version and various services require "real-name" verification which requires a passport, etc. Even further some services require real-name verification from a Chinese national.
In addition, you must have an ICP license to use most if not all of the services they offer within mainland China. While Alibaba offers ICP registration, it is a partially political process and the requirements can change at any time (and can be different depending on who you are or who you talk to).
You must also consider the likelihood that your data and your users' data is exposed to the Chinese government. We only use Aliyun to serve users in China and are careful about what kind of IP we store there.
