This is actually a complex subject. Go has mutable objects yes. But it also doesn't treat everything like an reference type. You have to send a pointer as the message if you want the receiver to be able to modify the object you sent. (slices and maps are the exception to this) I actually don't think immutability is as key to concurrency as it gets hyped to be. Clojure which I've used and enjoyed does tend to get in your way with the immutability by default.
This to me makes Clojure less relaxing. In fact Clojure is not relaxing it's challenging which can be enjoyable but I wouldn't say it's what I want to do everyday for my job. Go I could easily see myself doing it everyday.
As I said Go has a lot of good qualities all in one package. Clojure has some of those qualities but not all of them.
The pointer to the compiler api is interesting. I hadn't seen it before. I don't see any way to serialize a source tree in there but that might be just for lack of searching. I wonder why things like gofmt and company don't exist in the java toolchain already. I mean outside of third party IDE's.