I bought one a few days ago (weird coincidence), from Amazon for around $35 at the advice of a friend. Skeptical also, my 97% level upped to 98-99 with deep breathing, and then I held my breath as long as I could possibly do it and it dropped to about 93-94.

That's obviously not a confirmation that it's calibrated or trustworthy, but at least it shows a relative measure that seems real in both directions of oxygen saturdation. Surely if I have a drop due to illness I should notice, even if the number isn't exact.