It is important to get the right fit, if they are too big they will rub and be uncomfortable, the toes must not (or barely) touch the ends of the toe pockets. It is easier to stretch them if one foot is slightly bigger than the other - push a pencil and tissue tight inside the toe you need to stretch, or clamp them to a DIY table and steam with a kettle or leave for a couple of days. It really works.
I agree with everyone else, go easy! walk around the house for the first few days and then try rougher terrain, do not run too far to start with, build up slowly. Running on concrete is the hardest on the feet, but after a few months, this will be no problem.
I was very stiff in the ankle area every morning for months, but that passes. The only time I do not wear them now are if it is very cold with snow on the ground, but I have just ordered a pair of vivo barefoot hiking boots for the colder months, they have lovely boots too. The VFFs are very uncomfortable if they get wet and cold, even the FLOWs which are supposedly warmer with 5mm soles instead of 3mm, so I always take a spare pair with me when I hike and change them half way through, just so I can get a little more comfortable and dont start to get blisters.
In summer I wear sprints (and classics for dressier occasions), and love them, though be prepared for the comments. They never fail to start a conversation with people on the street.
To start with I had cramps all along my foot arches, sometimes really bad, but this passed after about 3 weeks and I guess it was because I was starting to use muscles I hadn't used since I was a child. I also got my little toe caught in a pram wheel which was really sore, so you have to be aware of your toes getting caught in things like branches etc.
The overall feeling of freedom I have now is amazing, I love walking and hiking up in the mountains with them on feeling every stone underneath my soles, gripping every root and bolder....it is wonderful.
I do not plan to buy any regular shoes ever again.
There is loads more info on the Birthday Shoes site, about purchasing and fitting VFFs and other barefoot shoes, people's experience with them (with loads of photos) and news on new products - they are planning on bringing out VFF boots this year, but I think they look hideous ;)
there is even a forum where you can ask questions and swap one shoe with someone else if you have feet that are two different sizes.