I've been waiting to respond to this thread until i got around to touching up my REX45 and K390. Until now, in my knife purchases, i had prioritized sharpening response over higher edge strength and wear resistance. My "higher end" steels until now were CruWear, LC200N, HAP40 (not used much so far), s30v, VToku. I am excited about REX45 and K390 in large part because i understand them to be relatively easy sharpening for how bad@ss they are!
While i like my regular stainless (S30V, VG10, Sandvik) and ultra stainless (H1, LC200N), I also really like tool steels and other steels that are made to prioritize other characteristics over stainlessness. Horses for courses, as they say.
I have had REX45 in my pocket for work (remodel carpentry/ construction) and on my own time (mostly in town, some in the woods and mountains as the pandemic allows) for a little while now. My REX45 knives are a Native LW which i've had since early April and a Manix 2 LW that i got at the end of August. I carried the Native some, but have carried the Manix more. In fact, the Manix has had a lot of carry time since i got it. I have a handful of Native 5s and other members of that family, but this was my first Manix.
I have been quite impressed with REX45 for it's strength and wear resisitance. I do not have a great need for ultra wear resistance, but i do really like very strong and stable edges. One anecdote to follow:
At work, while installing a tongue and groove hemlock ceiling, i was shaving a board which had been cut out around a light can, but needed more space and was already fastened in place (d'oh!). While shaving the board to create clearance I accidentally jammed the edge with some force into steel light can hardware (d'oh! again). I was like, well crap, looked at the edge and couldn't see any damage at all!
For me this is what steel like REX45 is mostly for-- for making up for human error. I do like to try different steels out, largely because i am a very curious person, not so much because i need them. It is nice to find the steels that suit my uses best.
I suspect that some of my other favorites like CruWear and LC200N would have shown some damage from the same treatment. REX45 is growing on me. I have high hopes for K390 too.
As for corrosion resistance, i have been pleasantly surprised by both REX45 and K390 so far. I have carried the REX45 much more-- including in my front pocket for a ~20 mile/ ~5000 vertical ft. hike this past summer and daily for a month or more at work in late summer. I have not found any signs of corrosion yet. I do keep the knives dry and have wiped them down with mineral oil here and there.
As for grindability-- I haven't done any reprofiling, thinning nor repair. I have touched up the Native a couple of times on the brown/medium SM rods. I just touched up the Manix for the first time this morning. I have to say that REX45 touches up quite easily. I did buy some CBN rods for when i need to do more serious sharpening of harder steels, but i have not needed to use them yet.
I just acquired my first K390 in the form of a Delica-- also my first Delica.
I have barely used it. I have high expectations. This Delica is ground thin and it was sharp when it arrived, but i touched it up a little bit on the brown/medium SM rods when i touched up my REX45 this morning. It responded wonderfully to just a few passes on each side at 30 deg.
I will add more comments with more use if i find anything noteworthy.