The Vistaprint cards are fine. I have some for my very occasional freelance work, though I opted to have ones without the Vistaprint ad on the back. They still cost very little and arrived in about a week.
Vistaprint and anyone else offering things for 'free' hopes that you will be delighted with your sample product and return to buy something. I've had loads of things 'free' from Vistaprint over the years - they send very regular e-mails with offers - and I have paid to upgrade to larger quantities, photo uploads, that kind of thing. Be careful when going through the checkout process... it is very clever how it tried to up-sell you, and also sell you other things, including a monthly membership scheme. Read pages carefully, being careful which buttons you click.
Quality of the products is excellent. Some free things do contain a small vistaprint logo but most don't (think it was only the reverse of the free business cards).