I've never used an interior designer, but in Sydney there is Greg Natale who won Interior Designer of the Year. The website shows some stunning places and really diverse looks, but I'm betting it's not cheap.
Brilliant question, and I am looking forward to the answers you all submit :) I have a friend who lives on the North Shore who is setting up her own interior design business who assist me. Otherwise I a fortunate to have the design skills myself :)
I have never really thought about an interior designer, and I only have 2 friends that have used one. One was very fancy but impersonal (they had no books in the house - except one coffee table book - I didn't know people still even did that!) - it didn't do much to impress me with the benefits of using one. The other however, did an amazing transformation, making the house seem light and airy, and somehow larger, even though no renovations were done. They used Colour Plan Design Interiors (http://www.designcolourplan.com.au/home.html). The transformation did get me thinking....