Been using the J-MAX for a couple weeks now and I also had the initial concern about knocking off the magnetically held base as right off the bat, that thing was flying around my coffee area. Two things I've noticed...

1) With just a little bit of use, I've now slightly adjusted how I hold and grind, and no longer an issue in general. You just get a "feel" for it. That alone pretty well eliminated the concern.

2) The grinder comes with a little silicon ring and I found my favorite place for it is directly above the seam where the base interconnects with the main grinder body. So it also now acts as a sort of "guard" and guide for hand placement.

Have not knocked off the base since and would not want to go back to a screw-on base now.

As per grinding, loving the grinds and the ultra-fine adjustment. No experience on the "K" side of things so just can't comment on it, but I almost find the J-MAx 8.8mic adjustment is almost overkill for me...I usually find I am using 2-3 clicks on every adjustment anyways. I would probably give up the adjustability for convenience, functionality.

I have the K-max (basically the same as the J-max in regards to the catch cup). I've only knocked it off ONCE, and that was completely my fault. I laid the grinder down on top of the cup and didn't whiggle it to make sure the magnets "locked" it in place.. then I picked it up to put it away and the cup dropped onto the tile floor and I dinged the bottom edge of it (basically the idiot mark where I look at it for the next several years and wince because I was an idiot and dropped it). It's VERY strong and unless you have massive hands it's not coming loose under normal usage.

although, I'm still tempted to get a JX-pro to compliment my K-max or the super cheap K-pro (due to black friday) as a backup to my K-max lol

I think the J-Max or K Max series actually improves the magnetic cup so it can only be pulled off by pulling it straight down.

ONCE or maybe twice I've knocked the bottom of the K-Plus catch cup off while kinda dancing around, not really paying attention and vigorously grinding it hit my big fat tummy (not really) and flew to the ground. BUT if you're aware of it being a possibility you just need to conscious of how you're grinding and keep it away from objects or your body. It's not a big deal.

I remember seeing someone's review and they criticized the J max for the catch cup being more annoying to remove and I responded I think that's actually a feature, I think they realized it can get knocked off and came up with a solution / extra safeguard for that which is pretty cool.

So how do you find the K-Plus Compares with the JX-Pro or J-Max for those who have tried both!?
From what I've heard the J series is more oriented for espresso (smaller steps also help there) maybe more fines and body.
And the K series is best for pour over with higher clarity. I found the K-plus does make really nice espresso too though. I haven't compared with J series.