Ah the old nil values boxed into non-nil interfaces. Even after 8 years writing go code almost every day this still bites me occasionally. I've never seen code that actually uses this. I understand why it is the way it is but I hate it.
The advice I've read (and follow) is always to return values, not interfaces, from functions and test for nil against them. That IME tends to nip the majority of nil interface problems in the bud.
That works until 1) you don't want to export the value types 2) the return values aren't simple structs but slices or maps because []x is not a []X even if x implements X.
This affects discoverability, though. Your unexported type won't have public documentation. So you end up having to publish an interface anyway (even if you don't return it) or document in words what the method set looks like.