I have an oxygen concentrator. I bought it very early in Covid, before most people had heard of coronavirus. (They doubled in price about a month later. It pays to panic early) I got a Dedakj brand one, which is a cheap Chinese made one. It has actually gotten a fair bit of use, for both Covid and non-Covid related reasons.
My thoughts. I paid $250 for mine in Feb. 2020. Even the cheap ones are around $500 on Amazon these days. Though there seem to be some refurbished ones available for less. The 'cheap' Dedakj brand concentrator lasted for about two and a half months of regular use before it started making noises, and started to sometimes work, sometimes not. I have been unable to source replacement parts.
Would I make the same decision again? I'm not sure. I guess it depends on what scenario you are prepping for. For using it for a few weeks to get through an illness, the 'cheap' one is probably fine. If you think you may need it on an ongoing basis, I would bite the bullet and buy a better quality one. It is an expensive enough prep, particularly for something that has a relatively low likelyhood of being useful. Though I did end up being glad I had one. I haven't decided if I will actually spend the money to replace mine now that it has become unreliable.