I've used one before, but for my personal use. My sister who works in the fashion industry told me to contact a Stylist/Shopper by the name of Kiera from MyStylist Paris. She is a native english speaker, and really knows her stuff (I'm pretty sure she works with weddings too). She's worked for a lot of big name magazines in France and UK (which kinda put me off at first) but she was really lovely and normal.
When she did it for me, it first involved a phone conversation, and I gave her a few details about myself, then we met a few days later. When we went shopping, she had arranged appointments at each shop. She had visited the stores the day before, and selected clothes which she thought would be good for me. She was brutally honest about what suited and what didn't, and made me try things that I wouldn't normally wear. Within 4 hours, I had all the clothes I needed for the season, and spent less money than I would if I had gone shopping myself. We had such a great day out, and would recommend this to everybody.
Compared to other personal shoppers, she was around the same rate but each penny was well spent.
Al