The Rani was such a great character. When we first meet her in "Mark of the Rani". She's really not evil, she's just a scientist doing research. She just didn't have much regards to human life. She thought she was better then the humans. They were lab rats. That really doesn't make her evil. It's kinda like how we test things on animals. It would have been nice if her character was used more, but it was toward the end of the show, so it's hard to blame them.
In my opinion, the Rani had one of the best TARDIS console rooms ever. It was beautiful. It even had, what looked like the rings from Superman II instead of the column in the center of the console. Come to think of it, it's the only TARDIS owned by a female we have ever seen. It shows that her eye for decoration was much better then the Doctor, Master and the Monk. I wonder how Romana would have decorated her TARDIS?
Kate O'Mara will be missed dearly by the Doctor Who community.