Even though it has an amazing story, SE still has to make CoP a requirement for new stuff to get people to do it.
The story introduced in Chains of Promathia is really good. I mean really good. Its a shame that most people never get to see it too. That's a good thing because the story is about the only thing you get with this expansion. The problem with Chains of Promathia is that everyone wanted new jobs introduced but Square-Enix said no. Also, almost all of the new missions are annoyingly level capped and several of them are really freaking hard, requiring possibly 10 attempts or more to beat. I myself got lucky on 3 of these extremely hard missions, beating them on my first try. Worse yet as that most people get stuck on the first actual fighting mission, the Promyvions-Mea, Dem, and Holla. Even more frustrating is that you can't get to the Tavnazian Archipelago until you beat these. Being capped at level 30, they take a lot of coordination to beat. They have been nerfed to be easier but only the actual level, the bosses are still just as tough. It was never really hard to get to the bosses anyways. More frustrating than that is that you need to be on Chapter 4 at least to do pretty much anything new that's being added. That's a pain in the rear end because you have to do 2 of those extremely hard missions to get there. And Square-Enix is still adding even more new things that require you to be far into CoP to access, such as the new quick teleport to Ru'Aun Gardens, which requires you to have a title obtained during CoP mission 8-3. The reward for completeing the CoP storyline is one of three rings, a damage dealer ring, a mage ring, and a tank ring, that pretty much any job that fits the description would wear from the minimal level (30) all the way through 75. In fact I can't think of any job that wouldn't wear the ring if its for their job type description. There is also another reward when you tie up the loose ends from both the Rise of the Zilart and Chains of Promathia expansions in the form of the Ethereal and Magnetic Earrings from beating Apocalypse Nigh. Fortunately, as Bibiki Bay has gotten -quite- old, Square-Enix has pretty much abolished Chains of Promathia as a requirement just to level up by releasing Treasures of Aht Urghan with all its nifty areas that start earlier than you will ever go to Bibiki Bay at. Annoyingly Square-Enix has made it a requirement to be on Chapter 4 to get the new Diabolos summon and to enter the new Dynamis-Valkurm, Buburimu, Qufim, and Tavnazia zones and 8-3 for the Ru'Aun Gardens quick teleport, as stated earlier..
Though it all seemed extremely annoying while I was progressing through it, in retrospect, its one of the few challenging things present in Final Fantasy XI, and I feel like I'm a notch above your average player for actually taking the time and effort to complete it. And it really isn't all that hard, the majority of it is being frustrated by bad luck (for the record the paragraph before this one had been written before I finished CoP, so you can see how real the frustration is. For that very purpose, I only edited it so very slightly.) But most people who don't actually do the missions are just lazy and not putting forth the effort.
CoP has the same graphics from the original installment except that everything looks more real, especially noticeable at Purgonorgo Isle, just look at the water. Its so blue and full of life. They added a lot of cool looking armor and weapons too.
CoP has some kick ass battle music and the music in general added to CoP is very nice. Almost every CoP zone has music too.
In the past I would've said that SE pretty much forced this expansion down our throats, and they still are to an extent, but they have let up. In addition to that, I have discovered that CoP does give a nice challenge to the game and has the best storyline of all the expansions so far. In any case, you wanna get it if you play FFXI. Luckily Square-Enix has made it more readily available by selling it from playonline.com, before this move the expansion was -extremely- rare, especially on PS2, which still lacks any kind of Vana'Diel collection like both the 360 and PC have.