It's not a needle, more like a thin filament. Painless to apply, don’t notice it while wearing. Video explanation:
https://www.stelo.com/en-us/how-it-works
Underarm seems best for avoiding compression lows during sleep.

I used PRE-Meal BG to determine timing of meals, adjust macros, which leads to healthy and sustainable weight loss. If you only check post-meal BG, that is limited data. I’ve been using a simple glucometer for 4 years and the fingersticks never bothered me, but these new CGMs are popular for their ease of use and information.