What's your first impression of Eureka?

[Fyce]

The leveling is too long

It's actually not. Once you hit Lv.20, you're done forever. At least until they raise the level cap (if they even do it). The elemental level is shared by all jobs, so you only have to level once. Except if you die a lot without getting a raise, obviously. I made 72k EXP in two hours yesterday. That was enough for a level. I expect that time to increase a bit, but not much, as people will keep being able to spawn higher and higher Fates.
Compare that time to the other kind of leveling systems there is in FFXIV, and you'll realize that the Eureka one isn't that bad.

The leveling is boring

This is subjective. I find leveling my other jobs in PotD to be very boring and brainless. On the other hand, I enjoy leveling in Eureka, for multiple reasons. I'm not saying that this is the pinnacle of fun by any stretch, but grinding Fates in N.Thal, getting Atmas, doing books, gathering light, going A1S again an again, doing years old content, and so on... isn't what I'd call "fun" either. I'm just aware that I'm playing a mainstream WoW-like MMORPG, which helps not raising my standards too much. If I wanted true novelty and innovation, I wouldn't be playing FFXIV. So, I'd say that people's standards and expectations are part of the issue.

Getting crystals is too long

I just hit level 13 out of 20. So, I'm still in the leveling process, not even close to being max level, and I already have the equivalent of 3k crystals. This is enough to have a complete armor set of +2. Or almost 3 Anemos pieces. Or two +2 weapons and some letfovers. Or... you get the idea.
Once level 20, I think that Eureka will become my favourite place to go to gear alt-jobs. Plus, I like quite a lot of the AF3 gear, so, that's a nice bonus.

Killing the same trash monsters over and over again is boring and braindless

Call them trash if you want, but they won't hesitate in one-shotting you faster than God Kefka if you are careless. Besides, almost all the "trash mobs" have different kind of attacks. Some cleave, some have ranged AoE, some petrify you, some apply a potent poison, some jump all over the place... Even if it's small, you have to adapt to them in some way. The terrain also plays a part in this.
Sure, it's still farming trash mobs. But a lot of FFXIV is actually exactly that, would it be dungeons, fates, PotD...

All there is in Eureka is spawning NMs over and over

Same as above, all NMs have a different strategy that you have to follow. Especially if you go on higher level Fates. And while people compare them to rank S Hunts, the fact that the trash mobs are so dangerous changes the way you approach the fights quite drastically. Also, you are killing them at a very quick pace, all things considered. When I do some Fate trains, sometimes it only takes 10 minutes between the death of a NM and another. The average seems to be between 15 and 20 minutes though.
Maybe one of the reason why I like fighting them is because I'm a tank.

The rewards are bad and already outdated

For you main job? Yeah, surely. For you alt jobs? Eureka is probably one of the best content to gear them right now. Looks good though. Isn't glamour the true endgame anyway?
Although I'd agree that the chests are quite annoying. When you get back to camp with a hundred or two to open, only to know that the fast majority will be trash, you'd wish for the rewards to be better. Or at least useful like Eureka potions, a raise potion, some cristals and whatnot. I blame RNG too. Still no mount and no chocobo barding. Sadness.

Losing exp if you release is crap

If there was no penalty for dying, Eureka wouldn't feel threatening at all. There is danger because the consequences of your death do exist and are harsh. And I absolutly doesn't see any other method to make you feel that way.
Another thing to note is how people have responded to that mechanic: players are actually helping each other. They will raise you because they expect you to do the same for them. Since there's absolutly no competition and that the most efficient way of doing things is to cooperate and be helpful, well, people are actually doing exactly that. Frankly, it's been a while since I entered an instance with so much positivity all over the place. You may not like that mechanic, but you cannot deny the good things it generates.

It will die like diadem

Frankly, this is up to SE. If they keep updating it with new rewards and keep the thing living. But, as I already said, it's already an effective method to get some not-so-bad gear for alt jobs while tomestones and raid equipement is still somewhat precious. So even if the numbers drop, it should still be populated enough.

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So, overall, I think it's fine. Really.
It's clearly not some mind blowing content and it obviously has been overhyped (I still think that people are somewhat responsible for their own expectations though)... But it's not a disaster either. The map is great, the feeling of danger is somehow refreshing and brings good player interactions, the monsters (both trash and NMs) are interesting enough. While the rewards are lacking, gearing my alt jobs is enough to keep me interested... and hopefully SE will update it right in time when they need to.
It sure has its flaws, but saying it's 100% garbage is dishonest.