Lately, Darko has started up conversations on wired IEM vs wireless IEM. While I completely agree with his assessment, I don't feel that this goes for all types of bluetooth. My experience with bluetooth is in two ways, bluetooth to dac/amps and bluetooth to headphones.
Bluetooth dac/amps are pretty common these days. Fiio Q5s and BTR3 are what I own. The BTR3 is just a Dac and receiver while the Q5s has a dac/amp with Balanced outputs. Both solutions have different applications and quality differences. The Q5s takes the bluetooth signal, converts the audio and puts each channel through its own amp to the balanced output. In practice, it works really well. I love my Q5S, but bluetooth isn't the only reason. I can also take the unit and plug it into my computer or my iPhone and use USB if I want. I usually do with my computer, bluetooth with my phone mostly. I have noticed no difference between using the bluetooth and wired connection to the source, at all. USB tends to be quieter and I need to turn up the volume a bit more, bluetooth takes my phone's volume and the Q5s volume so I have more options (usually 70% phone and 50% Q5s). While IEMs can be annoyingly sensitive to the source of signal, I feel that IEMs can't fully incorporate a proper DAC into them at the size they are. Having a dac/amp I feel is the best of both worlds. As I said, my home set up and my mobile set up using the same headphones (Sendy Aiva this week) have ZERO difference in quality that I can find.
Bluetooth headphones have one thing wireless IEMs don't have, space. look at the Sony WH-1000xm3, big with a good battery life and space to have ANC and a dac. I have the MEE Audio m6 pro on a wireless mmcx cable, and the quality doesn't compare at all to the Sony headphones. The space that a full size over ear headphone allows is giant compared to the little BTR3 I have, its the size of a tic tac container. The Sonys are as big as my Sendy headphones! With all that space, they got a nice battery with DAC and ANC processor in easy.
While I am not saying that the future can't change, at this time no "true wireless" or ibuds will ever hold a candle to a actual full size headphone. Sony WH-1000XM3 headphones are widely called the best ANC hi-fi headphones currently and I recommend them. I'm just going to stick to my corner and use my Q5S with my Sendy Avia's. Because I'm special.
(photo, as always, taken from google images.)