What do you think about custom designed plugin/theme options UIs?

You probably noticed the trend of making custom designed plugin/theme options UIs, especially in the commercial ones (on CodeCanyon for example – custom boxes, tabs, accordions etc).

I personally believe that UI should be unified in the first place and that is really easy in WP to achieve using some core’s default HTML structure + CSS classes => following the styling guide.

However it is maybe sometimes not enough and that’s why why there is that trend. Another reason could be “branding” or just some “coolness effect”.

What do you think about this?
Do you prefer some custom designed UIs or the default one and why?

P.S. The main reason I ask this is also because these days I started with plugin development and I trying to make this decision of which path to choose.

Custom UI is great where it improves the experience and makes the task easier.
After all, a plugin extends WordPress and therefore a lot of the time is extending the user interface. This might mean a screen that is arranged completely differently to any other screen, but if it is logical and understandable then there’s no cause for confusion.

Where custom styling is being used to ‘brand’ a plugin, it’s just annoying and naff.
Custom UI should respect the colour choice of the admin backend.

I much prefer plugins that could be mistaken for being part of the core, and that should pretty much be the goal.

So, respect the core UI but don’t be restricted by it.