Here's a great demonstration on the difference of smoothness between each frame rate, and the exact problem Apple has to deal with. Chances are unless you're reading this on a gaming PC, 60fps and 120fps look the same, as your monitor is only capable of displaying 60fps. You just sort of need to imagine the difference between 60fps and 120fps as the same jump between 30fps and 60fps:
This ultra-high refresh rate of the ProMotion display once again elevates the iPad to something that feels like science fiction, as most things in iOS have since gained a level of smoothness that almost feels fake- It's like when you see a trailer for a video game and are sure it's just one big pre-rendered cut scene and the game turns out to look just like that. It's a weird sensation, and one you're going to need to experience at an Apple Store.
This new 10.5" size is also one more iterative step to perfecting the iPad. Last year I picked up the 12.9" iPad Pro and while you'd think the giant screen would be amazing, the device ended up being just too large and too heavy for my tastes in day to day use. The 10.5" iPad Pro is the perfect compromise between the iPad Mini being a bit too small and too close to the Plus-sized iPhones and the slight bump in screen size makes it feel like you're getting a lot of the benefits of the larger pro without the extra size and weight.
Of course the new iPad Pros have gotten faster once again, with the new A10X Fusion chip sporting a six-core CPU and a 12-core GPU. Per Apple, this results in a 30% performance boost to the CPU and a 40% boost to the GPU over last year's model. The problem here, much like , is that while 30% and 40% seem like big numbers on paper, the previous generation iPads aren't devices anyone would describe as "slow."
With Apple still selling tons of non-Pro iPads which sport the A9 chip, and iPad Mini's which feature the comparatively ancient A8 chip, aside from the few "pro" apps featured during the keynote there just isn't a ton of motivation for developers to specifically target these top-tier devices. This just leads to things likely loading faster, but again, it's not like apps were slow on older iPads.
There's an argument to be made somewhere that maybe the increased horsepower of the new iPads is required to render games at 120fps, but it really remains to be seen how many developers end up adding 120fps support. From speaking to a few different iOS developers, it should be reasonably easy to update games to allow them to take advantage of the new ProMotion screen, but no one is sure just how much more battery that's going to burn. 3D games rendering at 60fps historically have been major battery drains, so doubling that frame rate might be problematic if you're not plugged in. Needless to say, I'm very interested to see how this evolves.
512GB as a top-tier option is a welcome addition as well. Admittedly, I'm likely a bit of an anomaly as I download practically everything that gets released on the App Store, but even with 256GB between apps, music, and movies I was always deleting stuff to make space for more. I'd like to think that 512GB is enough to have difficulty filling up, but, who knows.
The truly strange thing about this year's release of the new iPad Pro is it almost feel like they released them too early. It's not a surprise that third party software support for the ProMotion display is nearly non-existent on day one, but most of what Apple pitched for these new iPads during the WWDC keynote is all the incredible new multitasking things you can do between manipulating files and using the new dock to rapidly switch between apps. None of that is here yet, leading to the feeling that you've got this new cool thing that doesn't really do a whole lot of cool new things just yet.
But, hey, it's the new iPad, it's better than the old iPad, and the new screen is super awesome for apps that support it which right now appears to be limited to only the core Apple apps and a small subset of third party apps. If nothing else, it's worth checking out at an Apple Store just to see the huge difference even simple things like scrolling in Safari experience when rendered at 120fps. I'm not sure I'd rush out and buy one right now though, at least until either iOS 11 launches or more developers update their apps.