I've played up to the Evil Forest to see what the port is like. I really love FFIX, and when the PC version was announced I was really hoping that they would do it justice. Thankfully it seems they've done a good job.
The port seems to perform really well. Even my laptop (a 2010 Core i3 with a ATI HD5650 graphics card that was pretty good at the time but is showing its age now) managed to run it smoothly with only minor hitches (while I couldn't run the PS1 version in an emulator at the original resolution without hitching and badly stuttering sound), and even manages to display intensive attacks (like when Steiner uses a sword attack on Blank that releases a swarm of oglops) without the slowdown that was always in the original.
The graphics are actually quite impressive with the new character models. The fact that the polygon jittering has been eliminated means that higher resolutions are actually viable now and allows you to see much more detail. The graphical effects in battle also look really good. FMVs also look much better than the low-res compressed ones in the original game. The backgrounds are still the original resolution of course, which does look rather jarring, but it's not like they could have easily done anything with them considering that they were originally made in that resolution.
The game does have some problems still. The battle interface is too big due to it being ported from the mobile version.
As you can see here, the health bars especially are further apart which obscures much more of the screen. I ended up holding the Square button during attacks to get a better view, but I hope someone can find a way to mod them to be as small as they were in the original.
Another thing I didn't like was that there are some minor but inexplicable changes (such as the ATB ready sound effect and the battle intro swirl being different). As I've played the original a lot they were very noticeable to me, but I don't think they really make much difference overall, and anyone who hasn't played the original won't care.
It has 360 controller support and button prompts. There is some DirectInput support but it doesn't support the Dualshock 4 natively and doesn't recognise all its buttons. I ended up just using DS4Windows. It also kept moving left unless I unplugged the controller until after I launched it, but I don't know if that's just because of some Windows 10 update. A more serious problem is that the analog stick doesn't actually do analog movement, which is annoying and really needs fixing.
All in all this is actually a great port with a few issues that should be fixable. I'd definitely recommend it to people who don't own or have never played the PS1 version. If you already have it on PSN (unless it's the PAL version of course) it's only really worth getting if you want the extra convenience of the speed-up features, or to play it on a big screen (because it definitely looks better than just upscaling like on the PS3).
EDIT: It doesn't mention it anywhere, but there's an auto-battle function that you can get by pressing the RT button (or J) during battles. It doesn't speed it up though, which would have been useful.