I sorta blame @ChibiBlackSheep for introducing me to a VERY old version of this idea. Basically, the ability to pair your phone to a device that plugs into your car's cigarette lighter/power port, it will then transmit on FM to your radio (for audio streaming and hands free calling). This is handy for people who don't want to upgrade their car radio (such as, I want to keep the appearance of a factory radio). Another friend was recently talking about these and I decided to hop on Amazon and pick up 4 different ones to sample! This is NOT a fully comprehensive review. I am just reviewing some items of interest (and in some cases an item does not pass a priority check and I did not spend more time on it).

No idea of the brand names attached to these, but I am looking at from left to right T25, T81, Sonru BC80 and the T85M. All units are under $20. You cannot upgrade the car stereo for $20.

I have had about a week with the BC80 (it arrived first and then I ordered three more to try). For me a BIG item was how well audio quality was for the person I call. Items like them hearing an echo of themselves on delay or sounding like I am distant are negatives. All testing was done in my Grand Am, in my driveway, engine running, heat blowing at a low blower speed and using 88.5FM. Any observations of music sound quality. After that, can I listen to music off MicroSD/USB and have an incoming call interrupt it. Then ease of use, appearance any note worthy items. I honestly did not pay attention to anything on the device's display once it was configured for 88.5FM.

BC80 - Admittedly got more testing just because I have had it a week. My test caller claims it sounded distant and he could hear himself in a delayed echo. I found I had to turn the volume up on the radio pretty high to hear the caller as well. In testing of the other three this would have been a stopping point, but since I had it, I tried it more. Buttons easy to use, has the ability to turn off by holding a button (honestly, if your power port does not turn off with the car, I would suggest unplugging the device). If you listen to music on SD, it will remember not just the song you were on, but the time of the song (sorta cool). Has a BASS enhancement, I personally found to be lack luster (I like how my radio is set up, the enhancement sound bad, so I turned it off). Multiple color displays you can set or no LED lighting. Call quality is it's only real fault.

T85M - Dual mics but my test caller said I sounded distant. This limited my testing. BASS enhancement was just like the BC80, did not sound good to me. Multiple LED configuration options. Skip/Rewind was controlled by twisting the **** on it. For me, I was not too impressed with that navigation (but may vary based on vehicle). Multiple color displays you can set or no LED lighting.

T81 - My second dual mic test device. My test caller reported that it had much better sound quality from the previous two! No echoing of his words and I did not sound as distant. This was a success. I wanted to try all units that supported music on MicroSD, the T81 claimed on the Amazon page to have this feature but it did not per the package and the the device lacks the slot for it. I admit, I have not tested music on USB yet (not sure if I will or not). Easy to set up and easy navigation buttons. It has a couple LED light mods that are really lack luster (basically a multi-color display or non lights). Audio from my phone sounded good. No power button or power off function, must unplug.

T25 - My test caller reported that the sound of my voice was great, no echoing of his voice. He said it was marginal, but he felt the audio quality was better than the T81 (but they were both very close). While I was testing this, my father called. He had no idea I was testing with a device like this until I told him. He said the audio quality to him sounds great (he became a tester and didn't know it lol). Also easy to set up and easy to navigate. If I have music playing on SD card and someone calls, it will interrupt the SD music for my caller. No power button or power off function, must unplug. When you plug it back in, it does remember if you were last using Bluetooth or SD audio. Not sure if this is a negative or not, but when you power it back on it restarts the last audio file it was playing. Their is no option to turn off the LEDs or change color (they are on all the time and they are blue). In my grand am, it is impossible to read the digital display as the **** is in the way unless you flip it upside down or duck low to look at it. But, I see really no need for that once you have the device configured. I feel the sound is amplified, really comes through clear and loud.

For those not wanting MicroSD, all devices can play music from a USB drive. You can buy some physically small USB drives like these SanDisk 32gb units:

So far, I am a bit torn between the T81 and T25 units. I think the biggest issue is the T81 lacks SD Card support and the T25 lacks the option to turn off the LEDs. I want to do a little more quality testing (such as validate that the audio play back is true stereo, something like Bohemian Rhapsody is good to help prove that). And just get a little more time with the two units.
There are TONS of other units like these. My problem is how to truly weed out others to find the ones worth while.

Also, it looks like the USB/SD playback does not have a shuffle feature and does not care about folder structures. I dealt with this on another device years back (it played the files in the order they were written to on the disk). To solve this problem, I created a script where I store the MP3s I want on the drive in a folder (all in one folder, no sub-folders), point the script to that folder. It will then append a 6 digit random (non-repeating) number as a prefix. Once completed, copy those files in there new alphabetical order to the drive, you now shuffled the music.
And for those that have heard about 5 years ago, I have been working on a project with an Arduino so I could use the radio's native controls to manipulate a MP3/Bluetooth device. I still want to do this and got pretty far with the project, but I have other items needing my time. These devices look like a good option at the moment.

I know this was a bit lengthy, but I hope others found this of value.