After a couple of months of owning the JMJ Mustang, I feel like I can objectively review the bass now.
First of all, let's put it out of the way, I love it.
Out of the box, it came with Fender 9050 Flatwound strings which complement the vintage sound and feel of the bass. In terms of setup, the action was perfect and there were no dead spots but the intonation was off. It needed some major intonation on the E string and the A string needed some minor intonation fixes. Both the D and G were properly intonated. The fretboard needed some oil as it was a little dry but those were all minor things in my book. The gig bag it comes with is just a regular gigbag for a normally scaled bass. Not suited for a short scaled bass but it will do the job if needed.
On to the bass itself. It is pretty lightweight and it feels really good on the hands. I know that the relicing process is not for everyone, but I can assure you that the neck on this thing feels like it has been played for 50 years, it's perfect, feels like a real vintage bass. It comes with vintage lollipop hipshot tuners which are welcome (those are reliced to fit the image and feel of the bass) and looks cool and the Seymour Duncan pickups are, in my opinion, the biggest selling point of this bass. They sound really defined and punchy. Coupled with flatwound strings this bass provides a deep and rich sound with great definition. Very P-Bass like, but with its own character.
The tone knob is very responsive and has a lot of range, you can hear a difference even with a minimal change.
I was looking for a bass that channeled a vintage feel, that was easy to carry around, but that sounded big at the same time. I wasn't disappointed, this bass ticked all the boxes and it will surely be with me for a long time.
Secret tech: This is a great pick/mute bass! The thing that surprised me the most in this bass was the thump you can get when mute picking. I never heard such a beautiful pick attack. It's perfect for those boogaloo fast runs.
Summing up:
The build quality is topnotch, don't be discouraged just because it's made in Mexico. There's attention to every detail and it sounds like a P bass but bit more focused (exactly like how JMJ described it).