I have it in my hands since 2016, purchased from China, through a large foreign website.
The first thing I did was a "surgery" and removal of the fake battery it had. I connected a battery from an old Nokia 3.7V.
All these years it plays continuously in the car, (Taxi), through the phone without any problem at all.
The sound quality is NOT what I expected.
It is MUCH better.
I cannot have an opinion on its microphone because I was not interested and I never used it. I wanted it purely and only to listen to music and internet radios.
I am happy that I found it in a Greek store now, and at a much lower price compared to the approximately 15€ I paid back then.

For the outrageously low price (2-3€), it is obviously worth it as it essentially makes any speakers wireless.
4 stars because in some songs, for a reason I don't know, it makes annoying sounds (static)

Very good product for its price. The battery is satisfactory, although compared to other Bluetooth products, it is not the best. The only thing I do not recommend is the microphone. Not good at all

The device works perfectly today, it connects directly to my phone and the sound is satisfactory. After 7 months of use, in the fifth month, it started acting up and the sound was not clear, just like its battery that drained very quickly, something expected for its price. It still remains five stars because for its price it is fantastic, although I would recommend getting an FM transmitter which is at the same price and you won't need to charge it, just plug it into the car's lighter and you're ready to go.

What you expect for 2€. Very cheap construction (the interior moves with the slightest movement), but it works fine.

Immediately after the first connection test, the main button would sometimes catch and sometimes not, and generally never connected again..... money wasted

5 euros that I bought in 2018 and still works today, it deserves 55 stars for such a small amount (the price of a coffee). Not like those 1-2-3 products that many people buy without realizing what they will get, this microphone is not bad at all. Its battery is great for long trips, lasting more than 4-6 hours. It performs incredibly well at what it was designed for, which is to listen to music hands-free with good sound quality. And if you're a bit resourceful, you can even add a 18650 battery or an even larger one, and never have to charge it again. I remember using a 3200mAh 18650 battery and it never ran out of power

I bought three of these, for 5 euros each, and after a while they all broke. The internal batteries they have to function are cheap Chinese ones.

It worked very poorly, only 4 times, and even then with difficulty. And just after less than a month of having it, the charging port broke

Average sound quality
Average build quality
Better to spend more money for something better

I tried the 4 euro product for the car to connect it to the aux in order to listen to music from my phone. If it lasts more than 6 months, it has paid off its cost. So far, it hasn't interrupted the sound.

I bought 6 of them, out of which 4 worked and lasted for over a month, 3 of them. It does its job very well, but only until it breaks. It's really a hit or miss if you'll get a properly functioning one.

For 5 months now, with almost daily use, I haven't had any problems. The sound is fine (for the price, it's quite good). Don't expect something fantastic, it just does its job... The negative reviews are obviously from those who thought they were getting a Bluetooth receiver for 50+ €

If I could rate it with -5 stars, I would have done it. This particular product seemed very good to me on the first day, as I was listening to music wirelessly in my car, without constantly holding a phone with a cable. However, on the second day, it constantly needed charging, and then it stopped charging altogether, and not only that, the little connector for the AUX broke. Unacceptable, don't even think about buying this unless you're willing to spend €5 every two days.

I put it in an old amplifier and now it works great wirelessly. Of course, I had a little difficulty getting it to work, but it was probably the amplifier's fault! For 5 euros, it is perfect.

The downside is that every time you close the car, you have to press it to open it, it is not automatic. Otherwise, it is worth it

I recently acquired it, I press the button and it doesn't turn on. Does anyone know what's happening?
P.S. It is fully charged.

Poor construction quality. My button came off, but I glued it back on. It does its job. The music doesn't lose much of its quality and the calls are good, I would say, with the windows closed, of course, depending on where you place it. It's a product that you have to buy to see if it's good or bad. The price is very low and that's good. It also has uses that you couldn't imagine before buying it. For example, I bought it for the car, but I also use it to make my room speakers wireless or some wireless headphones.

Monophonic sound.
Tiny battery that lasts very little and needs to be almost constantly connected to the USB of the car's audio system. With this money, I didn't expect anything better

I bought it today and placed it on an old home cinema that was lying around. So the whole system now became wireless.
It really does a very good job and I bought it for only 5€. I didn't encounter any problems at all. I still don't know how long the battery lasts because I just received it and it showed 50% on my phone.
I'm deducting 1 star for the following simple reason (that no one else has mentioned). When you have it turned on and you go to charge it, it shuts down. Ridiculous.

Update 24/1/2020: I am very satisfied with the Bluetooth. Not a single problem. The battery lasts about 3 hours with almost full volume and around 4 to 5 hours with medium and lower volume.
I bought 2 more. For now, everything is good. The only difference they have compared to the previous one is that these 2 can charge and play simultaneously, but they produce interference in the speakers.

Perfect!!!
Great range of 10 meters, definitely good transmission without any loss.
I will buy 2 more for stereo and car.

For its price (4.5 euros + shipping), it is excellent. It connects instantly to mobile phones, tablets, etc. without any problems or disconnections. It has very good sound quality (again, considering the price) even at high volumes, and overall it is a very good purchase for those who are tired of cables and extensions to listen to music at home. I have it permanently plugged into my sound system (AUX) and whenever I want, I connect my phone or tablet and it plays from wherever I am. It also gives you the option (through your phone) to choose if it will also function as a Bluetooth headset for calls or not. I just hope it lasts for a long time and doesn't break in 2 weeks like a similar one I had bought

It connects easily with any device. The sound is decent - average.
Give it moderate volume to avoid disconnections.

This converter does what it's supposed to do. The battery lasts for about 6 hours, it has stereo sound and clear audio quality, it saves you from cables, it's stable and I haven't had any disconnections so far, and IT SAVES YOU FROM BUYING A BLUETOOTH SYSTEM!!!! YOU SPEND 6 EUROS INSTEAD OF 50+

Unfortunately, this specific product is defective. It frequently loses connection, resulting in interruptions while listening to a song, cutting off three times. Additionally, while driving, you cannot reconnect it. Therefore, stay away from it.