Ironically, the more you can make it feel like I screwed up just as I do when throwing discs in real life, the more fun it will be! :)
OK, the back 9 is really neat and challenging and the physics same WAY more real (almost too real, damn it!), but I had some issues last night that made it a little frustrating. The first thing I suggest you add is a way to jump straight to a hole, or at least jump to the back 9 directly so we can test that in a more focused manner. Maybe it's already there, but it would also be nice to have a "repeat hole" option in case you want to really test the heck out of one particular hole.
First, the little display that shows the overhead view of the hole was completely missing (the signs were just empty pieces of wood) from all the back 9 holes.
And for some reason, the popup version was just displaying the overhead view from one of the front 9 holes (see photos). It was very much not obvious as to where I was supposed to go on each hole when I was half way to the basket; I was frequently confused about where to throw. Yes, the fly-through on the hole helped a lot, but what would be really, really, really helpful would be a dot on the mini map (ignoring the fact that the bug I experienced hid the map from me entirely) with your current location AND the direction you are currently facing, just like a car GPS. Now, I forgot to consult my watch very much, so maybe you kind of already did something like this there, but if so, that might need some explanation.
I also fell through the course "into the void" a number of times on one hole where the disc would fall into a very small pond of water in the middle of a hole and the respawn put me in the void every time (see photo).
Once I had the issue with not being able to grab the disc but it eventually glitched a few more times and let me grab it again. I think this was somehow related to being on a slope. I think you need to build in some kind of buffering or different movement detection (block movement when only turning is happening vs. actual lateral player movement) because randomly sliding down slopes is also disconcerting and can make it frustrating when you're just trying to grab a disc out of the bag and you suddenly slide down a hill, looking up at your floating disc bag! :)
I know this is not possible and probably just related to my tiredness when I tried the course for the first time last night, but part way through the back 9, I felt like the physics changed and my discs were really not going where I wanted them too. It became a tedious game of throw, press A, throw, press A, faster and faster in frustration. Hopefully that was just me being weird and the next time I try won't be like that.
On an unrelated note, what's with the clunking around noise that you hear when you first enter a course? It's not horrible or anything; I'm just curious to know what it's supposed to be.