Different QR Code readers will have different success rates. Most phones with a QR Code reader will be able to easily scan simple QR codes with phone numbers or web URLs embedded in them. However, for QR Codes containing a vCard, we'd recommend a phone camera with at least 5 megapixel resolution, due to the density of information contained in the code. Basically, the more information you add to the code, the harder it'll be for the code reader to read it.
We've tested our QR codes on all the major phone platforms, and with most of the leading phone models to ensure compatibility. However, older phone models (iPhone 3GS and below, older Blackberry models) may struggle to successfully scan a complex QR Code due to their low resolution cameras.