A bunch of people I know buy nail polish exclusivelly from those 3 sites and are happy with them. I think the only complaint I've ever heard was about them quickly running out of stock on some of the most "in demand" polishes and I remember reading about some issues with delivery time (up to three weeks to Europe).
Personally I buy from TransDesign only, as they do
accept Paypal and I'm totally paranoid about using my credit card for online shopping. I'm very happy with them, the communication is great and you can't beat the prices