Really Lovinit! It only took 20 minutes to install and plug her up. The tone controls are in a configuration I'm not used too. (easy fix) Sounds really strong and articulate.

I've now installed both the Trickfish J-Core and John East Jazz preamps on different basses. There are obvious similarities between the two, so it's impossible not to compare.
The J-Core is super easy to install, just like the John East. No soldering (though you do need to use an eyeglasses screwdriver for each), and yes indeed the battery fits nice under all the electronics. The quality of the electronics and the wiring is extraordinary in both offerings. You'll need to use the eyeglasses screwdriver again to manipulate the EQ switches in the J-Core, but you do get a massive amount of tweakability.
The J-Core definitely wins out on the most important factor in a preamp, and that's TONE. Whereas the John East brings a famously dark tone to the party, the J-Core just transfers your original tone (IMHO) and makes it eminently flexible.
Where I lean toward the John East, is in what I call the ergonomics. The EQ switches are all REALLY tiny and very close together on the J-Core. I'm 54 years old, and it was a bear trying to see what I was doing, then using that small screwdriver to manipulate the switches. Also, the control panel has 3 pairs of stacked knobs, and it's REALLY hard to move just 1 knob independently. They could fix this with knobs where the top knob is considerably smaller than the bottom ... plus the feel on these pots is almost like speed pots. Might want to try "stickier" pots.
Overall, a great preamp, but with a few considerations could be perfect.

Flexcore on a Jazz plate, that's basically what it is. Powerful, transparent, and extremely customizable with the dip switchs. Sounds mean with the DiMarzio Relentless it's paired with. Also, installing it is really a breeze. Very happy with the J-Core.

I'm still getting the hang of it, but I'm able to dial in some nice tones so far with my single coils. I'm excited to try it with some humbuckers soon!

I could not be happier with this replacement preamp. The tonal options are amazing. Definitely able to dial in a sound where the bass sits very well in the mix. I will be buying more to use in other basses.