I've got about 8,000 hours logged into FFXI (actual playtime, not idle time) across two characters. I did a bit over an hour of Eureka tonight with some friends, many of whom also played XI for a good stretch of time.
I loathe it. I can, and did, run around and uncover the map after we finished - got one-shot a couple times which was amusing the first time but then the novelty of it wore off and the reality of that set in - but that's about the only fun or amusement I had. There are tons of flaws present, not the least of which being that the game just wasn't designed with this level of sustained combat in mind, but generally it actually doesn't really remind me of XI very much (despite countless efforts on Square's part) and it doesn't mesh well at all with what XIV is. There are a number of things present in Eureka, and by extension XI, that I've always felt should have been present in XIV's overworld - more aggro types, day and night cycles for monsters, overworld objectives that make the world feel alive (running around Eureka seeing people fighting all over the place was nice) - and if this "experiment" leads to those things being implemented, then that's great.
But Eureka itself isn't good. I think a lot of it can be attributed to needing to do the "Eureka" content (read: level up. A lot.) first before you can probably realistically make a good dent in your first Relic, but that both assumes that people will want to do multiple Relics (almost none will) and doesn't do the Relic itself any favors effectively having a grind locked behind another unnecessary grind. And we're assuming that when they expand Eureka they'll also be increasing the level cap, right? So it may not even be a one-time thing.
If Eureka was what ARR itself was at launch, with everything being slowed down a ton and there being all manner of support systems in place (like XI itself had), I'd be okay with it. But I can't really justify, nor do I feel any compulsion to, effectively play a shallow XI-inspired minigame within the content cycle that XIV has. The fact that the gear is so far behind the curve from Day 1 just makes it worse.