Pros:
Down Under is the classic wooded course. And when I say wooded, I mean REALLY wooded. Whatever course you're thinking of that's really wooded, this is more. Every hole puts all your control skills and endurance to the test nearly every kind of technical shot imaginable. Mistakes are punished severely.
Every hole has some different lines available. Unlike some wooded shots where there's only one way to get to the basket, this course usually has several. Holes are short enough that touchy turnovers often work, but there are opportunities for skip shots and even sky right-to-left shots on hole 2.
The elevation here is amazing. In addition to the holes that play up or down, there are several steep drop-offs, usually to the right side. While it does make for lost-disc opportunity, you always have to be careful and possibly choose a safer line. One bad kick can cost multiple strokes when a shot that pures the tunnel will be on the green.
Despite many holes being fairly short, distance perception is difficult. The tunnels always seem longer than they are, so a full-power driver shot is often a little more than you need. Sometimes it will be really hard to park the hole from where you're at, but a fairly short putt isn't too hard. This makes shot selection important when you really don't need the most aggressive line to get up and down for the birdie or par. One example is hole 6, in the right pin position: a right-fading shot might park it, but might skip down the hill right, while a straight shot is nearly guaranteed to leave you with a longer, but makeable, putt.
Cons:
Despite many holes being really unique wooded holes, it certainly gets old and leaves this course with relatively little variety. You won't find yourself pulling out your furthest-flying discs too often (though if you do, they're probably sailing down a few hills). It means that regardless of your skill level, this course won't have anywhere near a comprehensive test of your skills. Some might say the Up Top course is a good compliment to this one, but I would rather some holes play up the hills and into the open.
Upkeep is not good, and the way the course is makes upkeep nearly impossible entirely. The tees are natural and get muddy easily, and a lot of fairways are treacherous as well. Signage is practically nonexistent, and even having a map won't always solve everything. Being in the woods, you can usually just follow a path to the next tee, but the main space to get confused is around the baskets of 10, 12, and 17, and the corresponding following tees. That area is a little more open, and you even have to walk across the fairway of 12 to get to the tee of 17, potentially without knowing it.
There are a few holes that are not well designed at all. Maybe they were okay when the course was originally installed, but they're certainly not good now. Some examples are hole 7, which has a dumb fairway shape, 8, which is too tight of a line, and 13 and 14, which have little going for them other than the dropoff on the right. Those two are pretty much the same anyway. A few others, like 1 and 17, are a little boring and will leave every drive with a putt, provided it makes one gap. Now, while learning the gaps you need to hit is an important part of this course, parts of it fall flat pretty quickly.
Other Thoughts:
WATCH YOUR DISCS. Rollaways are a common occurrence here, and even anything going slowly is still in jeopardy. There could be steep slopes where you aren't expecting them, and the rocks can make discs quite unpredictable.
While the poor tees are a problem, the ruggedness of this course was intentional, and it's meant to stay that way. In many ways, it helps the feel of the course and makes it a little more exciting, even if it technically makes it a bit worse. That said, this course is by no means neglected. The bridge on hole 8 was replaced recently, and a staircase to leave the course has also been added.
Several holes have two tees, but unless you really want the extra variety, they don't add much to the course. The alternate tees on 1, 4, and 8 are nice, but I always play the right tees on 5 and 10. Hole 17 has two tees, but I think your choice is dictated by the current pin setting - again, there's no signage helping you out there.
Some pins are hard to see from the tee. This isn't usually a problem - the baskets are bright orange - but if you're expecting the basket to be somewhere it isn't, it won't be fun.
At the end of the day, plenty of people will love this course, and that's okay. Other people will hate it, and that's also okay. While it is a great test of skill, there are obvious downsides and many better alternatives for wooded courses. But this place isn't at all bad, so it doesn't deserve a low rating.