How to measure Tobin's q? How do people calculate Tobin'q empirically? 

Does anyone use it to make investment decisions? In particular, the version I learnt has no company debt involved with only capital under consideration, will this affect empirical research?
If you are interested in Tobin's (average) q,  you might find useful A Simple Approximation of Tobin's q (Chung and Pruitt (1994)). That paper has a highly accurate approximation for Tobin's Q using Compustat. While the JSTOR copy I linked to is pay-walled, other versions appear not to be.