Everyone living in China seem to have strong opinions about Astrill. Many love Astrill and many hate Astrill. I will only talk about my own experience with Astrill. If you want to read the comments of disgruntled customers, affiliates, and former employees just search around on Twitter or Reddit.
Astrill is the most popular VPN for China expats which can be good and also bad. It's good because they will always make sure the service is working good in China even if you need to buy extra paid add-ons for protocols that work good (StealthVPN and RouterPro). The bad part about being so popular is that they seem to attract more attention from the censors. Astrill was recently mentioned in a Global Times article. Having your VPN service written about in a newspaper controlled by the communist party of China is not a good sign of things to come. It means the authorities are paying close attention.
From my own experience I will say that the StealthVPN and RouterPro protocols work extremely well and everything else works very poorly or not at all. With RouterPro I got up to 20Mb/s speed on my router which is very impressive which is double the maximum speed of 10Mb/s I can get with OpenVPN (due to the limited CPU power of a router compared to a computer). I should mention that the RouterPro is a propriety protocol developed by Astrill. We don't know the details of this proprietary protocols but closed-source software is questionable when it comes to security.
If you do sign up with Astrill make sure that you at least get the StealthVPN add-on or the Pro addon if you are using it with a Router. You will be very disappointed with the service if you don't get these add-ons.
If you an Astrill customer and having uninterrupted VPN access is important to you then I recommend having a second VPN as a backup in case Astrill stops working for you, or you want to connect more devices, or Astrill bans your account (this seems to happen often). A good second VPN for Astrill customers is VPN.AC because it's cheap, works very good in China, allows you to connect 6 devices at the same time, has excellent customer service, and is less popular (less popular means less attention from the authorities).
Tips for using Astrill