Nielsen Norman GroupEvidence-Based User Experience Research, Training, and Consulting
Usability findings derived from a broad base of diverse studies have higher credibility than those based on many users with a single stimulus.
Users generally prefer designs that are fast and easy to use, but satisfaction isn't 100% correlated with objective usability metrics.
User testing can be done anywhere; witness our international studies, carried out with equipment that fit in a carry-on bag.
What's worth the most: field studies or user tests? Depends on your company's usability maturity, but user testing is the safe bet if you can do only one thing.
Heuristic evaluation involves having a small set of evaluators examine the interface and judge its compliance with recognized usability principles (the "heuristics"). Adherence to specific methods can improve the outcome of an heuristic evaluation.