I think it really depends on your personal circumstances, but there are a lot of benefits to employing an interior designer. I recently worked with Absolute Interior Design LTD, a company offering interior design in Newcastle and I found the main benefits of hiring them were:
If I hadn't employed an interior designer, I would have had to spend a long time liaising with various contractors and sourcing furniture, which I didn't have time for in my schedule. Instead, I just had semi-regular meetings with my designer, to check everything was heading in the right direction.
Although I pride myself on having an eye for design, my designer was much better at visualising what would work best in the spaces in my home than I was. I think if I'd have designed the rooms myself, I wouldn't be nearly as satisfied with the results, and would probably have ended up making a lot of changes the near future.