I was given a couple, but I've also had one for over 10 years and that's a long time, so a backup wasn't a bad idea. I also carry one in my purse, one with my computer when I travel and one in my checked baggage. That way I always have one when I need one. I use them daily to check my O2 levels.
I prefer a Nonin Go2 Achieve. They are more expensive, but I've owned four or five of them and they have consistently been accurate and lasted forever. They are medically calibrated and respond quicker to changes than any of the cheap models I've seen. This is important, especially if you're someone whose O2 sats drop when you start to move. You know pretty quickly just how low you've gone.
I bought a inogen 4 was the best thing I did works great and I get about 4 to 5 hours on 2 liters per battery they are exspensive so had to save awhile as some insurances won't cover it but makes my life alot better as I can be put and about alot more and longer
thanks now I know,time for a nap but i just had one three hours ago,alway seem tired latley.