I recently hired one. I saw her work mentioned in the media a couple of years ago and liked it, so I bookmarked her and recently got in touch via her website.
The ID I chose did 3 hour consultations as a start-off (price about £500 in London for this).
I did a lot of prep for the appointment - wrote out what we wanted to achieve, what our specs and constraints were etc - to try to get the best out of the time. I also had an idea of what I wanted to achieve following the appt itself, which was to be able to write a plan for dealing with various things - structural, decorating - that I had identified and then follow the plan to get the things done. The idea being that if I spent some ££s upfront on planning I would save later on not making mistakes.
I found the appt more than worth it, for the money. The ID looked around my house and made some much bigger picture comments about how we were using the space we had and how we could use it better, which had simply not occurred to me. Then she also made some very specific recommendations for particular rooms - paint this one this colour, change this door so it opens the other way, move this lightswitch.
After she left I spent a few more hours writing it all up and now we are in the process of actually following her advice. Great so far - and something else unexpected happened too, which was once the ID had made some design recommendations I seemed to feel less paralysed about making my own. So I am going to do most of what she advises, but not for example what she recommended for one thing, for example and I have made a bold wallpaper choice for one room.
Go for it!