I recommend the ZigBee protocol only when a design requires interoperability with other ZigBee devices, or when someone wants a ZigBee certification. For proprietary designs or basic wireless communications, modules that operate to the IEEE 802.15.4 standard will do a nice job without the software overhead ZigBee requires.
I was once very interested in those TI 802.15.4 SoCs, but those currently in production use an 8051 core and to do anything useful with them requires a heafy investment in development tools (eg IAR which costs 2-3k$). No thank you, ARM for me please.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.