I put some time into the demo on Sunday (4 hours, using primarily the Storm) once the disconnect issues were resolved and the hype died down a bit. I played on Hard, and encountered only one bug. Not a single disconnect was had that day. Here's my take.
Ground to air transitioning is incredibly seamless. It's wonderfully fun to jump in the air and just boost away to your destination. However, flying itself feels a little wonky, though this is supposedly not an issue with a controller. I used the KB+M control scheme, but I've heard positive reception with a controller.
Like everyone else, I started with the Ranger, then moved on to the Storm. It wasn't until I unlocked the Storm that things really took off (pun intended).
I wasn't impressed all that much by the game's gunplay. My bullets often lacked the feel of impact. Maybe this was an issue with audio feedback, but it felt more like firing a plastic gun than something loud and explosive.
Spells, however, were the exact opposite. Freezing enemies in their tracks then casting a massive lightning bolt was incredibly satisfying. Video and audio feedback was impressive on both sides, and it was remarkably addicting to chain them together as it often landed me a combo. Combo'd attacks mean damage bonuses. Spell cooldown was forgiving. There was a healthy flow of alternating from guns to spells, as I couldn't sit there and cast into infinity.
I didn't interact with the town inhabitants at all, but that's only because you have no reason to. I do feel that you move a bit too slow in town, and it seems odd to me that there's no sprint function. I feel like I'm slowly inching toward my objective markers when there's downtime, but that's a minor issue. I think the menus could use a good cleanup, it feels unnecessarily complicated at times. It took me a while to find out how to change my Javelin's armor color while in the paint tab.
The game didn't feel repetitive in regards to combat or enemy design. I'd see the same enemies, but no two encounters were ever the same. Boss fights felt relatively easy, but being in the air 90% of the time with a blink ability probably allowed for that. As a Storm player, I did take quite a bit of damage thanks to my glass cannon design and difficulty, and I think there's a major emphasis on being able to use your dodge/blink abilities consistently.
I ran the game on ultra settings, and had a smooth 60 FPS except when multiple ultimate attacks were executed at the same time, which dropped me down to 46 FPS. Aside from that, it ran well with few hiccups and no stuttering. I used Vsync with a Gsync monitor, and as such experienced no input delay.
For someone who had no knowledge of the game (I didn't know what a Javelin was until Sunday), I rather enjoyed it. There's a lot of potential here.