Ok, I started this off lighthearted and got swept away by nostalgia... Feel free to tl;dr this one its a wall-o-text.
I can summarize by saying yes it was that good and you can laugh at the following statement. 7 Years (with a couple breaks) and I had at the end one beastly geared lvl80 BLM. That's it! One lvl80 and I never finished ToAU or WotG missions. I'd say I completed 2/5 of the game's content.
The meat n potatoes for those who never played FFXI and feel like reading:
The Truth
It was really that good IF you liked complexity, story and cooperative play and didn't mind grinding. In my opinion what made XI so much better than the rest was story and depth along with consequences. You actually had to be not only good but in many cases stellar at it. One person screws up in Dynamis, Einherjar or dear god PM missions and all hell broke lose. And you had the reward of meaningful progress to show for it. Not just some Gil and a title but it literally unlocked entire zones and instances. There was a huge emphasis on crafting like XIV. Some of the best gear in the game was crafted and there was so much of it. There is simply too much to list.
Getting Geared:
For loot you had BCNMs, KSNMs, ISNMs, ENMs and more. All of which were single boss fights (no trash mobs) that you had to acquire the consumable items to get into. You got these through your regular play so when you had 30, 50 or 99 you could fight a single boss (10 min or so) and either get epic loot, crappy loot or fail. Not like the WoW format of endless Elites and a few bosses. In FFXI it never felt like you were grinding trash mobs in any endgame content (well maybe Dynamis). ENMs were hell and a half to get to sometimes but were open world and had interesting although frustrating gimmicks. Mount pomodorro (sp?) falling off the damn mountain or Brothers sliding down the hill and having to start all over.
Missions and content they unlocked:
The missions were truly Epic! I don't know many people who weren't ecstatic when they finally finished their Promivion Missions, got access to Sea and got their Raja's/Tama's ring. I know it brought a tear of joy to my eye. Zilart and Sky not quite so much but still awesome missions. Sky, Sea and Limbus for endgame was fun times too, 12-18 adults, we had drinks and colorful conversations while farming items to, That's right! Pop more bosses for more gear! Otherwise it would have been kinda boring. Then there were the 40 some odd missions of ToAU, Besieged, Campaign Battles (Way more fun when the cities still got zerged everyt 20 min lol) Assault instances, I don't remember how many there were.
The not so good:
There were a few major drawbacks like there are to any game. One was that the party system was by level range (3 levels) and with the exception of a few jobs BST, BLU, SMN soloing was not really a viable option to gain levels. So you had to get an EXP party to raise your level and at least one person bailed after an hour or less so it could take a long long time to reach 75. They added the level sync option and increased the exp easier mobs gave to combat this as servers lost population. This was great and should have happened at the 3-4yr mark rather than the 7th year or so. Another was the need for multi purpose players. It was hard to get into good active endgame LSs unless you had at least 2 lvl 75s and all of your subs + good gear though this was later on in the game. Also, like XIV there were many many NMs and most of the good ones were camped all the time by Gil Sellers, Lifeless players and underhanded LSs making them difficult if not impossible to claim. I never even went into HNMs since I never fought any unless you count King Arturo.
Conclusion:
All in all there were 100s of (Valuable) things to do at endgame. Because you could still do basically anything at any level as long as you had a level 75. The lower level BCNMs for example still had awesome drops that could be sold for tons of Gil and were as valuable as many high level endgame pieces. How many jobs are there these days? 20? I don't remeber but not many people could in the time the game has been live reach 75 in all of them, even then you had subjob combinations. FFXI was an endgame game. Nothing really got going until 75 but once it did it was hard to do everything, nay impossible if you have any social life or job.
I guess you could call this the manifesto of a FFXI crackhead.
P.S. That BLM was beastly geared ^^ MAB, ENF, INT and even healing sets. Novio, All HQ Staves, All +1 Crafteds, Oracle's etc. etc. (Never got my Herald's Gaiters though)