The IP Multimedia Subsystem (IMS) is the latest framework for a seamless conversion of the ordinary Internet with mobile cellular systems. As such it has the backing of all major companies since it aims to offer a unified solution to integrated mobile services, including mechanisms for security, billing, quality of service and so forth. We provide a unique assessment of IMS based on our experimental test-bed, investigating functional and performance capabilities. Our study helps evaluating the level of maturity of state-of-the-art open source technologies in view of the deployment of IMS. We assess the suitability of SIP, IPv6, MIPv6 and IPsec as basic IMS enablers, highlighting crucial shortcomings which need immediate attention.