How much should your development UI resemble the final product?

Except perhaps at bigger (or better) shops, the development UI is done by the developers. In your experience, how much has this impacted the final product, and how much time should we spend getting the development UI right?

From stackoverflow

• The interface you're building and showing to your customers is basically all they will ever see. You can sit down with them and talk for hours about what the product is supposed to do, but the number one best method to have the users spot problems or missing functionality, is to show them the interface as it will look like in the final product. You can do this with a prototype or the actual interface as it is being built.

  So I would say the interface should look pretty much like what you will be delivering at the end during development. This will guarantee that you get the right feedback, and that the user knows what he is getting. Of course, you can build an interface which doesn't exactly have the style of the final product (colors, window styles, fonts, etc...), but the functionality of the product you're building should be clear as early on as possible.

• I like to spend more time at the beginning of a project concentrating on the core functionality of the software. If you properly separate your business logic from your presentation logic (using MVC, for example), it should be easy to work on improving the user interface at a later date.

  I know some developers who will purposely make a user interface look different for a prototype than it will look in the final product. This is useful when you demo, making it clear to the customer that what they are looking at is only a prototype. One great example of this is the Napkin Look and Feel, a pluggable Java look and feel that makes it look like your user interface was scrawled on a napkin. Kathy Sierra wrote an interesting blog post on why you might want to do this.