Did some quick tests in all the various modes. Arena mode is incredibly fun. Setting megabeasts against megabeasts, sending swarms of 50 animals against megabeasts, elves vs dwarves, so many fun things to do with it.
In my new fortress I haven't managed to do all that much. The health care system seems intimidating, but seems to work ok once I realized I needed to actually enable the various healthcare labors on my doctor dwarf. I've broke into a large underground cavern filled with spiderwebs, fungi, mud and water which seemed to be the perfect place to expand my farms, instead of messing with irrigation. Of course the second I started to try and make said farm, a bunch of naked mole dogs came out of no where and starting running all around my fort. They didn't attack anyone but it led to what must have been a very comical scene of five dwarves with a slight bit of military training chasing after them in every direction until they finally caught them all about 5 minutes later. Speaking of that, the military interface seems very very intimidating. I'm sure it will give my a far greater level of control but it's going to take a while to learn. Can't wait to see what else I can find.
My only real notes from adventure mode are that the underground is big, very big. I didn't get to explore all that much.
Finally with legends mode the titans and forgotten beasts look great, just like everyone else is saying.
Performance wise I haven't really noticed any noticeable slowdown. A few of the menus seem to be slightly unresponsive, but it's perfectly playable on my end.