I recently got an android phone and with the power of technology I hope that added data on how my body is reacting to exercise may give me insights into what my fitness regimen may be missing. What should I be looking for, data gathering wise, in monitors and attachments? What capabilities and technologies are available for gathering and organizing bio-metric data? With accelerometers built into most everything step tracking should be pretty straight forward, but what about heart rate, blood oxygen, blood pressure, etc?
The gyroscopes, accelerometers, and the GPS you'll find in your phone or the basic armbands will only give you approximate numbers, and any app that uses the raw data will make some very approximate guesses as to things such as step length, so can be out by as much as 40%. Adding a heart rate monitor to the mix should reduce any errors in the predicted calories, VO2 figures to around 5%, and will give you something quantifiable to aim for, such as reducing your basal heart rate, or your heart rate while doing X. It will also allow you to approximate your VO2max figure.
Blood Oxygen and pressure aren't really relevant to general exercise, unless you have a pre existing condition, but blood pressure should drop with weight and exercise, over time.