For me this game is a part of new "Big Three" along with Dusk and Amid Evil. It was the last one I played, and after I made it through twice, on penultimate and ultimate difficulty, and found all secrets (oh God...), I think I am ready to make this review.
Ion Fury is an heir to the Build games - Duke Nukem, Shadow Warrior and Blood. My relations with these games are like this: love and cherish Blood and Duke, still re-playing them from time to time, never bored, always fun. Absolutely loathe SW, found it almost unplayable both back in the days, and much later in 20XX's when I started rediscovering old games, never made a playthrough and doubt I ever will.
Technically IF is a masterpiece of Build-engine engineering. Aesthetically it's a definite callback to Duke 3D, but gameplay-wise, I think, is much closer to Blood and Shadow Warrior.
If you want a lot of vocal and accurate description of the good stuff in this game go watch Civvie11, it's worth it.
My problem is, half way through the game I... got bored. And for a game that wants to be an heir to Blood and Duke that's like a death penalty. Around the levels where game introduces final enemies - pretty standard mutant zombies who come in packs, make your ears bleed with irritating sounds and have the sole purpose of draining your ammo, because at this point you definitely feel NO trouble with ammo management whatsoever. Yes, on maximum difficulty. No fun looking for secrets, no fun blowing up enemies - game burned out. In comparison, Dusk and Amid Evil grabbed you by the throat and kept you on your toes from beginning to the end. So did Blood and Duke 3d. Ion Fury... drags.
Overall, it's a solid, solid game. But of my neo-Big Three it's also the weakest one. An expansion is coming up this year, with a vehicle sections added, some new enemies and weapons... It has a chance to improve a lot, or to repeat all the same and make it even worse. We will see. As I said, this game inherited some big shoes to fill.