OST wise I still prefer original XIII, nothing beats Blinded by Light, Dust to Dust and Sunleth Waterscape lol but Crimson Blitz and The Saviour are pretty good I'll give you that.
As for the gameplay I love the technical masterpiece it is and the battle system has definitely changed for the better, it's alot more skill and strategy based rather than mash A/X and have dumb AI do everything BUT the spell/attack you want them to.
As for the whole customisation thing, I love it! Customising is my favourite thing in games even if it's just a colour pallete thing plus there's a crap ton of broad swords, gunblades, staffs and lances to choose from as well as Garbs referencing old characters, Garbs straight from old games such as Cloud's SOLDIER uniform, Aeris' Costume and Yuna's Gunslinger and Summoner Garbs as well as Lightning's very own Guardian Corps. uniform from FFXIII and her goddess armour from FFXIII-2.
It also has really cool and rediculous original Garbs lol that Moogle Garb xD, and asides from a few locked abilities set to certain Garbs the ability to interchange spells to customise your spells and attacks depending on what you need is really helpful and being able to wear ANY equipped Garb outside of battle is also really cool, and I have to lol at some of the accessories, I love that it ranges from hipster shades, to cat ears and a Moogle.
Open world, 'nuff said.
As I have said before it's essentially Classic Final Fantasy with a modern mix of Kingdom Hearts, if FFXIII was like this when it first came out maybe it wouldn't have had as much of the negative response it got :/