You won't hear me gripe much about the hardware. That 8.4-inch display is just big enough for the media and websites I like, and it's so high-res that the images occasionally have a painted-on look. Also, it's a pleasure to hold. Faux leather and metal notwithstanding, it's both comfortable and light. The iPad Air is undoubtedly svelte, but it feels a tad ungainly next to the (admittedly smaller) Tab Pro -- and that's before seeing the Galaxy Tab S, which is even thinner and lighter.
To me, the real clincher is Samsung's Multi Window support. While its absence isn't a dealbreaker, I'd love if every tablet had some variant of this going forward. There's an undeniable appeal to chatting with someone on Hangouts while a YouTube video plays. It's not as elegantly implemented as in Windows, since you can't easily restore a Multi Window setup if you lose it, but it does make good use of the Tab Pro's real estate.
If only Android apps took similar advantage of that display. Titles that aren't tablet-native generally scale well enough on this smaller screen, but it still feels like there's a lot of wasted space compared to the iPad apps I'm used to. Also, some of them don't cope well with the 2,560 x 1,600 resolution; buttons and other interface elements are occasionally a little too tiny for my liking. Still, these will hopefully get better as developers get comfortable, and they're not going to sour the overall experience.
Would I toss out my iPad and get a Galaxy Tab Pro or Tab S? Probably not. The iOS ecosystem is still stronger in some places (most notably games), and the Air I already own has enough screen resolution and speed for my tastes. However, I wouldn't balk if someone made me switch, and I'd have little hesitation recommending Samsung's newer slates to anyone who wants a lot of performance in a small shell.