Android 6.0 was launched yesterday, and Ars Technica has, so far, been the only outlet to give it a formal review. That said, it is a twelve-page review with a table of contents — so that totally counts for five or so.
The main complaint that the reviewer has is the operating system's inability to be directly updated. There is a large chain of rubber stamps between Google's engineers and the world at large. Carriers and phone manufacturers can delay (or not even attempt to certify) patches for their many handsets. It is not like Windows, where Microsoft controls the centralized update service. In the beginning, this wasn't too big of an issue as updates were typically for features. Sucker, buy a new phone if you want WebGL.
Now it's about security. Granted, it has always been about security, even on the iPhone, we just care more now. If you think about it, every time a phone gets jailbroken, a method exists to steal admin privileges away from Apple and give them to… the user. Some were fairly sophisticated processes involving USB tethering to PCs, while others involved browsing to a malicious website with a payload that the user (but not Apple) wanted to install. Hence why no-one cared: the security was being exploited by the user for the user. It was only a matter of time before either the companies sufficiently crush the bugs, or it started to be tasty for the wolves.
And Google is getting bit.
Otherwise, Ars Technica mostly praised the OS. Be sure to read their review to get a full sense of their opinion. As far as I can tell, they only tested it on the Nexus 5.