I've got my copy of the first Kickstarter material (Requiem, Monsternomicon, and the new Witchfire adventure) and I've read through the backer pdf of the second Kickstarter material (Borderlands and Beyond)
I haven't playtested any of it, and I'm not too familiar with the old 3e game material, though I do have a reasonable level of setting knowledge. The original 3e line kinda built a RPG setting from scratch which later inspired a tabletop miniatures game. The 5e material is going the other way, taking the tabletop game setting as its base and building an RPG on top of that lore.
Requiem covers the Warmachine tabletop game material, while Borderlands and Beyond is more concerned with the Hordes material.
The background and setting is covered in a pretty comprehensive manner without getting too deep into the weeds and minutae. Playable as is with no real need to hunt up extra background (which can be legendarily difficult considering how Privateer scatters their lore across dozens of supplements and (often-unfinished) fiction pieces which never get second print runs). The RPG line is set after the recent big upheaval in the setting, the return of the Infernals, and a significant proportion of the population jumping through a art deco clockpunk stargate to go into space in order to kick off a rather meh and uninspiring sci-fi miniatures game. However, more recent PP material from the miniatures game has talked about the return of the Orgoth and their new invasion of Khador etc. This stuff is not yet in the RPG, and has rendered a bunch of the first setting book 'canonically' obsolete, for whatever that's worth to you.
There are some significant lore gaps due to the way that the world has been divided into ~9 monthly book releases, and this is occasionally annoying. All throughout Requiem, for instance, we're told how something mysterious and terrible is happening in the elven land of Ios, but we are never told what it is despite Iosian elves being a playable race in Requiem. The Borderlands and Beyond release (still not commerically available) covers what's going on, but it's distinctly odd to have it divided between books like this. Similarly, there's big holes in the coverage of the Legion of Everblight, Skorne is basically not mentioned at all, there's very little coverage of Cyriss except in the history, and there's no playable gatormen despite the area they live sitting in between a whole bunch of regions/races who are covered. I expect this stuff to be detailed in later books, but it leaves big gaps in the line as is (although some is slowly being released as for-pay pdf-only enhancements). Still, you have the basics of the core Iron Kingdoms/Warmachine firmly covered, even if they've only scraped the surface of Hordes. They have announced an upcoming kickstarter focused on Cryx, which'll probably go live once the Borderlands and Beyond hardcopies escape from shipping hell (they've said they always want to have the previous kickstarter delivered before starting the next one).
As I said, I haven't tested the game mechanics, so can't comment on them too much (especially the more complex and iconic classes like Warcasters and Warlocks, the warjack customisation rules, or the heavy emphasis on guns). As others have said, there's an optional system for racial ability score bonuses that attempts to break the shackle between race and optimal class. There are some obvious balance issues - ogrun are waaaay overpowered as melee combatants due to their ability to wield a 2-handed weapon in a single hand. Other people have noted a major class-breaking exploit with Warlocks, but i'm not sure if that's been fixed in the final product after feedback. Some of the subclasses etc are interesting, some are clearly placeholders (invented in a spirit of 'we need a fighter subclass for this book, quick, think something up!' rather than 'oh yeah, [archetype X] is something we really need to cover due to its importance to the setting'). It falls a bit prey to the classic 3pp issue of inventing new core classes when they're probably not strictly necessary - gunslinger could easily be a fighter subclass, for instance. Overall, I suspect that mechanically, player options from this book would probably trend a bit (not overwhelmingly) more powerful, and probably somewhat less well playtested. If you're GMing, I'd be prepared to veto or house rule, especially if you allow non-core 5e material too. There are notes about how 'conventional' D&D classes etc fit in the setting - sometimes the answer is 'not well' but it's your game.