I'm 5'6, male, 28 yrs...my question: is 123lbs a good weight or bad weight?

Weight: It depends on your stature and your overall health. It is definitely thin, but thin can be lean which could be good. I would say it is neither good nor bad. Your body mass index (bmi) is between 19 - 20 which is more desirable than one between 25 - 30, let's say.

Check healthy range: You can use BMI calculator or chart to get an idea. BMI Categories: Underweight = <18.5, Normal weight = 18.5–24.9, Overweight = 25–29.9 & Obesity = BMI of 30 or greater. See: https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm