I bought my wife a Lily last christmas. Though we understood this wasn't a medical device and would be off by 5% or so, the main interest was the spo2 sensor to loosely monitor a condition she has.
Unfortunately, it has never worked in any consistent fashion. It fails reading most of the time, and is most often off by 30-40% when it does.
In order to force a reading she has to wear this device painfully tight, or press it hard against her wrist.
She's not overweight or overly skinny and has tried on both wrists and flipped below wrist, and followed all steps to troubleshoot. Loose is zero reading, comfortably tight is zero reading, try again later. Only reading is when too tight and painful.
Support has been less than helpful claiming since it reads sometimes, then that's good enough and meets their requirements for working product.
Does anyone have any special tricks to make this thing work? So far it's just money down the drain. The readings, when worn painfully tight did get better after version 10 software release, but still off by almost 10% vs finger pulsoximiter and still have to endure pain to get this in the neighborhood reading.