I'm an Android developer, and, at least in this platform, my opinion is that right now is necessary to make some changes to the default UI elements. Current Android UI is just too ugly.
BUT there are disadvantages in modifying it too much:
If the default UI is changed in later versions of the OS, your app can get obsolete in terms of how it looks, and therefore lose consistency. How much this affect to your app will depend on how deep are this changes.
In Android, there can be different default skins depending on the hardware (Sense, MotoBlur, ...), so you can make your UI look similar to some default UI, but maybe it's a lot different from other variations.
Users progressively learn how to use apps in a given platform, so it likely be more difficult to users to interact with your app.
Of course, all of this can be solved if you do a nice work, but it's not easy.
My recommendation is to make small changes, so users can still be familiar with all the elements, but making the whole thing good looking. Add some nice animation effects and you've got it.
For now, we've decided to make custom elements in our Android apps, and we've got pretty good results (Go!Chat Facebook as the last one).