What's the easiest way to create CRUD interface for custom entities? What is the fastest way to create admin UI for new entity type? Like at content types. I just need basic list/add/edit/delete actions. 

One would think this comes with entity api?

I have found a lot of solutions but all of them included a lot of custom code, which is prone to errors. 

is DrupalDefaultUIEntityController the answer? how can one use it?
The Entity Construction Kit (ECK) module allows you to create and manage entities through admin.
Here is a quote about it (from the module's project page):

...  builds upon the entity system to create a flexible and extensible data modeling system both with a UI for site builders, and with useful abstractions (classes, plugins, etc) to help developers use entities with ease.
ECK allows the creation and management of entity types with custom properties; adding bundles to entity types; and fields to bundles, with the help of the Field UI module.